Goldfish don't live very long in goldfish bowls. They thrive better in aquariums, artificial ponds or both. Fish bowls should only be used temporarily while their actual habitat is being cleaned or otherwise maintained. Sadly, this one is still widespread in advertising, as many companies still market small bowls as being for goldfish.
A baby turtle needs a lot more than a little dish of water with a fake island in the middle. Most aquatic species need deep water for swimming, lights to bask under, and some form of filtration.
Although betta fish can survive in small amounts of water, they will not thrive in it. Like goldfish, they're kind of messy fish, so they really need gentle filtration. (Or if you don't filter, you need to change the water daily, but that can stress out the fish.) And they really like to have space to swim around and places to hide in. As with the goldfish, tiny bowls (some no larger than a regular coffee mug) are marketed as being adequate for bettas. They are not. Also, they cannot be housed in the same bowl or tank as goldfish; most goldfish prefer much cooler water than what bettas need. (Bettas are tropical fish; they need warm water.)
Some owners provide their betta with a large tank, only to be disappointed when the fish stays in the corner most of the time. Some key things to check for: gentle filtration if possible, and lots of plants that will provide the fish places to hide and help to break up the flow of more powerful filters if a low-powered one is unavailable or will not work for the setup. Bettas can also be kept with some dull-colored fish, particularly bottom feeders like the very gentle cory catfish, if you want to liven up a large betta tank a bit.
To sum it up: Most aquatic pets require a lot more space than you probably think. Pretty much the only thing that can live in a space the size of the iconic goldfish bowl is a marimo — also known as a moss ball.
Animals and tails:
Almost no animals should ever be picked up by their tails. Most animals only use their tails for balance; tails usually aren't strong enough to safely hold the animal's entire weight.
Mice and rats may be held by the base of the tail and for no longer than is necessary, preferably only a few seconds (it's best to provide support under their paws with your other hand if you need to hold them in place by the tail). Not only is it painful, but particularly in the case of rats there's a risk that the tail's skin will tear under the animal's own weight and peel entirely off. This goes double for animal wranglers who allow actors to hold rodents in such a dangerous way during filming.
Some lizards' tails will actually break off if pulled on too hard, and then wiggle around frantically for several minutes after being detached. This isn't accidental—predators are likely to grab the lizard by its tail, and letting it pop off often distracts the predator long enough for the lizard to escape—but it's not a fun experience for the lizard either way. If you have a cat who likes to hunt, you are probably familiar with this.
In the case of mammals, the tail almost always contains part of the animal's vertebrae, so pulling on it or otherwise putting massive strain on it can have horrifying results for the rest of the animal's spine.
Holding a turtle by its tail, especially a snapping turtle, is a really bad idea. Not only is the turtle strong enough to escape your grasp, which may hurt it when it hits the ground, it can still bite or claw you if you're only holding its tail. If you must rescue a turtle in the middle of a road or pick it up for any reason. always lift it by the sides of its shell, from behind, and hold it away from you.
Although you still shouldn't pick it up by the tail, the West Highland White Terrier has been bred specifically to have a more robust tail than other dogs so you can grab it to pull them out of narrow tunnels if they get stuck.
Fictional accounts that depict pets of different species getting along, simply because they share the same owner, can be a potential recipe for disaster if the pets in question are, in their natural habitat, predator and prey. Despite what a story or cartoon might say, assume terriers, cats, and ferrets will always attack smaller furry animals, rats will always attack small birds or reptiles, and pond turtles will always attack fish.
People seem to be under the impression that it's perfectly good, or perhaps beneficial, for an animal that was born and/or grew up in captivity to just be released into the wild and let their wild instincts take over from there. This is a terrible idea for several reasons. For one, many animals, especially more intelligent ones like mammals and birds, need to be taught how to fend for themselves, and more importantly, how to interact with wild members of own kind. This takes very specialized training, and even then this is imperfect as we may not know everything about their behavior. It's also a terrible idea to release animals (or any organism) into an environment where they don't belong. Either they'll die quickly because they can't find suitable food or avoid predators, or, even worse, they might become an invasive species, which can have potentially disastrous consequences for native species and even the ecosystem.
Inversely, some people feel that a wild animal will always love and trust their owners if raised from a young age. This only turns out to be the case with some animals. Typically, the animal will only trust a select few people, and react wildly and aggressively to everyone else. Others will want nothing to do with people at all once they reach adulthood. So basically, leave wild animals in the wild. If you feel that you must intervene in a wild animal's life for whatever reason, promptly take it to a wildlife sanctuary or anywhere else where there are professionals who can handle the situation in the correct manner.
Most animals have a very short memory when it comes to understanding cause-and-effect, especially if the link isn't directly obvious. Media that depicts an animal feeling guilty after being scolded for something it did hours ago is complete hogwash. At best, most creatures will remember something along the lines of, "Last time I got in the crate, I went to the vet and that was scary, so I do not like the crate." In most cases, they will not understand why their owner is throwing a hissy fit at them after coming home and seeing that they peed on the carpet (so why do they look "guilty" when you come home and they've done it again? Well, if every time you come home, you scold your pet, the pet "learns" that you coming home is the reason you're mad at them, so that moment in time becomes one that they dread). If you're going to scold or punish an animal, the best time to do it is during the offending act or a few seconds after it; then move on, stop being (or at least, stop behaving) angry, and reward it when it does the right thing instead. Despite what the cartoons may tell you, your pets aren't going to agonize over their own bad behavior for very long, if at all.
Toys:
The yarn ball, the iconic cat toy, is actually incredibly dangerous to cats. Swallowing just a little bit of yarn can cause a fatal intestinal blockage — and of course, if you give them the whole ball, they're likely to swallow rather more than that.
Mirrors are an iconic fixture of bird cages, but they're not actually that good for birds. Worst case scenario, your bird will either choke on vomit because it's trying to "feed" the bird or it'll end up stressed/angry because it keeps on "attacking" the bird. Even if neither of those occur, mirrors make birds harder to tame because they prefer the company of the "other bird" over your company.
Frisbees and sticks are common dog toys in media, but professionals don't recommend them. Frisbees (especially human-geared ones) can get broken into pieces and accidentally swallowed, while sticks can be both a sanitary hazard and safety hazard.
A cat playing with laser toys is funny to watch, but many professionals don't recommend them because the cat can get stressed because it doesn't get the relief of catching something physical. String toys are better. And if you use a laser toy anyway, never shine the laser into your cat's eye or anyone else's eye because they can cause permanent vision loss and damage. Children in particular should never be allowed access to a laser toy — they are likely not to take this seriously enough and do it anyway, or may do it by accident.
Determining the age of your pet is not as easy as media often portrays - namely, you can't just use a strict math formula to do that, because all animals age differently compared to both humans and each other. Generally speaking, a one-year-old cat or dog can be considered to be the same as an eighteen-year-old human, while past that the differences vary so much between breeds an individuals that trying to determine things in terms of "animal years" simply doesn't work. And while there may be some modicum of consistency between dogs and cats, beyond that it becomes chaos. Some species are old at one year, while others don't grow up for several years. Take the misconception of "Dog Years" (IE: That one human year is equal to seven "dog years"), for example: in reality, there's no universal way to determine the age of a dog. How fast a dog ages depends on many factors including breed, size, and even the individual dog's family history. Likewise, smaller dogs generally mature faster but live longer than larger breeds.
Pets of the same species and opposite sexes should not be kept together unless the female is spayed and/or the male is neutered, unless you're attempting to breed them, because otherwise, the female will likely get pregnant or lay eggs (yes, even if they're related).
Foods and Feeding
Pets and milk:
Humans are perhaps the only adult mammals who can digest milk, and lactose tolerance is a mutation largely restricted to peoples who've had domestic cattle for a long time—as in, longer than recorded history.note And even then, lactose intolerance is the norm for the majority of people, even in those cultures that habitually drink milk. After weaning, the vast majority of mammals completely lose the ability to digest milk and will get digestive problems if they have too much.
Any time a cat is fed straight ruminant milk. Especially if it's a kitten. Cats are always lactose intolerant; some less than others, but any more than a very small amount will cause an upset stomach and diarrhea. Watering down the milk can help make it easier to digest, but it's not necessarily 100% effective. You can give your cat either goat's milk or special pet milk such as Cat-Sip available cheaply at pet supply stores (this is different from the milk replacement formula for kittens). Also, cream and yogurt are less harmful for cats than regular milk. So long as your cat doesn't throw up or display other digestive problems, you can give small amounts of regular milk as an occasional snack, but it's still not recommended. Some cats can eat cheese safely. Give him/her tiny amounts and watch out for diarrhea.
Same goes for dogs for that matter; dogs should not be given milk in more than tiny quantities. Being partial omnivores, some (very small numbers) canines can process milk, but most experience acute intestinal symptoms including gas, diarrhea or vomiting. That's because most dogs can't digest lactose well at all; others who can could only be given watered down milk—for example, 1/2 cup of milk & water is more than sufficient as a treat to large dogs while 1/4 of milk and 1/4 water for medium, while small dogs shouldn't even drink that small amount.
Mice and rats are also often fed milk by their owners, according to a book on rodent care: "...this is fine in small amounts like thimble sized cups for mice and bottle cap sized for rats for a once in a while treat it is alright if 2% or 1% milk." Most mice and rats are omnivores, and like their wild cousins they eat not only seeds, grains, nuts, berries and other fruits, but also worms, insects, fish and eggs — but milk is not a normal thing. In fact, lactose can give them gas and digestive issues just like anyone who is lactose intolerant (though, in mice, it can cause them to bloat and mice can't burp or fart), so while it seems OK, it's probably not the best idea to feed your mouse or rat milk even in the 1% grade.
Aside from mammals, birds and reptiles cannot digest milk and attempting to make them do so will cause them to suffer or die.
Onions, garlic, and anything else in the Alium genus are not a good idea to give to pets. Onions contain toxic components that may damage red blood cells and cause anemia in a wide variety of animals such as cattle, sheep, horses, dogs, and cats.
Chocolate:
Humans metabolize theobromine (a bitter alkaloid compound found in the cacao plant) much more quickly and efficiently than most animals, and for most animals it's very toxic and potentially fatal, especially if they get hold of dark chocolate. Most vets will flat out state that animals shouldn't have any, ever, no matter the concentration, just to be safe. See here for more. It's also possible for humans to get poisoned by the theobromine in chocolate, but this is generally only a real danger for the elderly. Healthy people would have to consume a very large amount of it for this to happen and would probably throw it up from eating too much in general before any negative side effects could set in.
British media at least can be excused for this somewhat. "Dog chocolates" still appear in UK-published pet manuals (and low-concentration chocolate like milk is only harmful in very large amounts). In other words, the dog is very unlikely to die from eating a chocolate-chip cookie or even feel mildly sick but a five-pound baking bar would be cause for concern.
Most media are aware that chocolate is toxic to dogs, but not cats. Cats are, in fact, even more sensitive to theobromine than dogs since unlike dogs, cats are obligate carnivores. Due to lacking a necessary gene, felines cannot taste sugars or "sweetness", and thus have no interest in such a sugary foodstuff. Interestingly, rats can tolerate even more theobromine than humans, but it's still unhealthy for them due to the caffeine content. Beyond the theobromine issue, the sugar in chocolate can give both dogs and cats tooth decay, just like with humans (and this is a bigger issue with cats and dogs than with humans, since obviously pets can't brush their teeth).
Alcohol and caffeine are much more toxic for a dog or cat than for a human. While dog owners may see the animal licking a small amount of spilled coffee or beer and assume it will be fine, in large quantities (an entire cup of coffee for a medium sized dog) it may kill the poor animal, while it just distresses a human. Hops in beer is also toxic. It's safer to never give the dog caffeine or alcohol.
Horses, however, can handle a bit of beer just fine, and it's often used as an appetite stimulant in racehorses (Guinness is the traditional choice). This is because horses are a) big (and produce plenty of the enzymes responsible for breaking down alcohol), and b) herbivorous, so will eat just about everything that goes into making beer anyway. It's not recommended, obviously, but a pint of beer will do your average horse no harm at all.
Anyone attempting to feed an obligate carnivore (such as a cat or ferret) on a vegetarian or vegan diet is all but guaranteeing that the animal will starve to death, even if it actually eats the food. Obligate carnivores need the proteins found in flesh to survive. A vegetarian or vegan diet can be pulled off for canines, but only with special vitamin supplements and close monitoring. Please don't try it without the help of your vet (and really, if meat-eating is an issue for you, consider sticking to herbivorous pets - it's just simpler and easier on the animals). One couple in Melbourne nearly killed their kitten while attempting to force the cat to go vegan.
Specific Animal Groups
Mammals
Rabbits:
Many illustration of magic tricks such as the old "Pull a Rabbit out of My Hat" trick show the magician holding the rabbit up by the ears. This act is quite painful, much like pulling a person by the ear is, and in fact old-school magicians hold rabbits like that because it's painful; when a rabbit is being held by the ears, it kicks and moves around, which is much more "appealing" than holding one limply and shows the audience that it's a real, live rabbit.
While rabbits will eat carrots, they should be fed sparingly if at all. Feeding a rabbit a diet consisting solely or mostly of carrots is like feeding a child nothing but candy bars; it will not end well. The bulk of a rabbit's diet should consist of dried grass hay (alfalfa is too rich) and a wide variety of fresh greens, of at least three varieties a day and varying which three (but not iceberg lettuce which has no nutritional value and too much phosphorus). The typical rabbit may require a few cups of greens and hay per day. Pellets should really only be fed to juvenile and pregnant rabbits, and anything else (including carrots) shouldn't exceed a tablespoon per day, even less for small breeds.
Hamsters:
Any show portraying hamsters (most particularly Syrian hamsters) living happily in pairs or groups. A normal Syrian hamster would eventually kill even a litter-mate, as they are loners by nature. Male-female pairs may occasionally work, but would lead to the female breeding continuously till she dies of exhaustion. Some types of dwarf hamsters can be kept together without incident, provided they've known each other from a young age and are provided adequate space so that nobody feels overcrowded. If kept in a tiny cage together, they'll still act just as aggressively towards each other as their Syrian cousins.
Really elaborate hamster habitats, with multiple chambers and plastic tunnels running throughout a room or even through walls, are virtually impossible to keep clean. A single hamster (the only safe option; see above) will choose two or three chambers as its sleeping place and larder, then visit the others only to use them as latrines.
Rats:
Rats are extremely social animals that quickly become stressed if left alone for long. Keeping a rat as a single pet is quite cruel, as it'll need at least four hours of contact with its human owner per day just to keep from becoming a basket case. There are some exceptions to this note Too aggressive towards other rats (as in, injuring/killing their cagemates), not accepting new cagemates (if their previous cagemates died——Usually the case with elderly rats), or health problems but, unless your rat is an exception or you have oodles upon oodles of free-time, get more than one rat.
For that matter, bread has caused deaths among pet rodents when it swells up inside the throat, causing choking. Bread should be toasted until it's crispy before giving it to small pets.
Cats:
Holding a cat by the scruff of its neck without supporting the hindquarters at the same time is bad. Mother cats carry their kittens this way, but it's generally advised that owners shouldn't even try it. Kittens held this way instinctively freeze so that they won't hurt themselves by squirming around too much. If you know exactly what you're doing you can gently pinch this area to evoke the same response in adult cats, but never actually pick them up by it. Adult cats, save for unusually tiny ones, are far too heavy, and being picked up like this strangles them. The key is that the hind legs need to be supported as well, but in most cases it's best to leave scruffing to your vet - an upset cat might thrash even if scruffed correctly, and thus can still injure themselves (and the offending human). You might occasionally see a member of a veterinary hospital's staff lift an angry cat by the scruff. Assuming the staff is competent, they are either A) very close to a surface to put the cat down on; B) unable to get to protective gloves and are taking the lesser of two evils if the cat might injure itself more severely if left to roam; or C) the cat is so highly aggressive that it absolutely cannot be carried any other way with the equipment available on hand. Whatever the case, please leave this task to the staff, as there's a good chance that an animal that fractious will injure somebody if lifted in the normal fashion.
Allowing your cat to free-roam outside is a hazard to your pet and the environment, hence why there's so much regional Values Dissonance surrounding it (with North American experts claiming cats are strictly inside pets, while European experts allowing outside cats with precaution). Domestic cats are an invasive predator known to hunt whatever they can catch and it's easy for a cat to get diseases, fleas, worms, and/or other parasites, or injuries outside. At best, cats should be collared or microchipped, neutered/spayed, and vaccinated before being let out. It should also be noted that a indoor-outdoor cat's average lifespan is around only 5 years or less, the same as a feral cat, compared to an indoor only cat's 12-18 years.
In the US, it is still unfortunately common and accepted to get cats declawed despite this practice increasingly being banned in many other countries. The reason it is such a concern is that it is essentially the exact same thing as amputating the last bone off a person's finger, and can lead to various problems like lasting pain or sensitivity for the cat when it walks, or even at first glance seemly unrelated issues, like urinating outside of the litter box (this can happen if it is painful for the cat to step on the litter, and/or if the cat is stressed because of the declawing). Most people get their cat declawed because they mistakenly believe it to be only like a simple permanent nail trim and an easy way to "solve" minor behavioral issues that can be trained out of the cat or managed, but declawing may just cause the cat to have other, often worse behavioral issues, like increased biting or anxiety.
Llamas are so fluffy and soft-looking that people who know little about them are likely to try to cuddle with one. This is a terrible idea, as llamas are defensive about their personal space and may spit or bite if manhandled. The same holds true for other livestock such as sheep, goats, cattle or pigs. As nice as it would be to cuddle them like a cat or a dog, they don't like their personal space invaded and interpret it as being attacked by a predator. Most horses will also not respond well to being hugged, although there are a few exceptions, so always ask the owner first.
Ferrets are very frequently mistaken for wild animals due to their scruffy, weasel appearance. While there are wild species such as the Black Footed Ferret, pet ferrets are no more wild than a Siberian Husky is a wolf and have been domesticated since at least Ancient Egypt. They're domestic animals who can't survive on their own however are frequently depicted in fiction as being able to. Even a few real world laws against ferrets mistake them for exotic pets.
Dogs:
They should not be scolded by shouting their name. This will only lead to the dog thinking its name is an insult, threat, or means "no".
Dogs should not be hit on the nose or slapped with a rolled-up newspaper for wrongdoings — this will only make the dog scared, and dogs are bad at learning when scared. Also, some owners literally rub their dogs' noses in their pee or poop as part of housebreaking — not only is this cruel, however, it doesn't work.
Dogs should only be hugged if they trust whoever's doing it very well, and even then they should be hugged from the side, not in front.
Never leave a dog in a car on a hot day — not even if the windows are open. Because the car insulates heat, and dogs have fur and so get heatstroke easier than a human, this could kill them. Also, if the ground is painfully hot to the touch, never walk a dog on that ground, as it could hurt their paws.
Birds
Old cartoon-style birdcages are terrible enclosures for birds. Birds are energetic animals that require lots of space to move around in and stretch their wings, something these kinds of cages can't supply. Additionally, old world birdcages can be made from materials that are potentially deadly to birds. Fortunately, use of these cages for housing birds has fallen out of practicenote nowadays they're only used for decoration and instead bird sellers typically recommend safer, more spacious cubic or dome birdcages or (if you have the money for it) large aviaries. Never buy one of those tiny cages except as a travel cage, and make sure it can be locked.
Most psittacines — parrots, cockatoos, cockatiels and parakeets - are extremely social. They are extremely intelligent, extremely social creatures and will go insane if left alone. Never have only one bird unless you are home all the time and prepared to pay lots of attention to him/her. Keep the bird in an area of your home where most of the "action" is. It's better to have two or more. Clean out the spare room and make it the "birds' room" and let them fly around and socialize in there.
A common, romantic depiction of raptors (eagles, hawks, falcons, etc.) is to have the bird sitting on the hand, arm or shoulder of its owner, ready to fly off at command. In reality, this is a terrible idea for two reasons. Firstly, it is an innate instinct of the raptor family birds to Go for the Eye and it is severely advised to not have them sit anywhere on your person but on your outstretched hand. Secondly, even on said outstretched hand, Raptor talons are designed to tear meat to ribbons—they can easily cut through human flesh and clothing. Most handlers wear heavy leather gloves to protect themselves. It's also very strongly recommended that you keep the bird attached to you via rope. You should never pet a bird of prey (with the notable exception of some hand-raised owls) either. They hate it.
It's quite common in media for birds to eat out of a human's mouth. This is extremely dangerous in real life, as human saliva is toxic to birds.
It's very common in media (and definitely Truth in Television) to see people feeding ducks and other birds bread. In reality, birds can't digest bread at all, but it swells in their stomachs and makes them feel full, with the result being that if given too much over a long enough period, they may develop nutrient deficiencies, or starve to death because they don't go looking for real, nutritious food. (This happens more commonly than you think - birds will often cluster around areas where multiple people or passersby's are giving them bread, so it's easy for them to get too much). In addition, it makes the birds associate the areas where they get fed with getting regular food; as a result, some of them fail to migrate in the winter, which results in them dying from the cold temperatures and lack of food. Never feed a bird bread. There are similar reasons why not to feed birds French fries either, with the additional problem that birds cannot handle large amounts of salt, as it can potentially result in kidney failure.
There is actually something called "angel wing syndrome" that is sometimes found in waterfowl living near human areas. It's basically a twisting/defect in one or both wings that renders the bird flightless, and it may be connected to nutrient deficiencies caused by bread consumption. Affected birds usually die early. It's permanent in adult birds, but in young birds it may be reversed by treatment and a more appropriate diet, without bread.
Another problem feeding fries and bread can cause is making birds overweight, which can cause health problems, or indirectly kill them by making them a more attractive target to predators (especially if slower and/or can't take off in flight as easily.)
An alternative to giving a bird bread is to feed something more species appropriate. It's usually more tasty to the bird anyway.
And speaking of bird diets, no, Polly doesn't want a cracker. Crackers tend to contain sugar, salt, and fats, all of which can be harmful to a parrot when fed in large amounts. Parrots would want fruits, seeds, nuts, and meat instead.
Reptiles and Amphibians
Lizards:
Lizards are not low-maintenance pets, and some species like iguanas can be very demanding. Regularly changing the water they use for drinking or doing business, maintaining the lighting they need to regulate their temperature, and giving them a fresh, balanced diet are just some of what you will need to do to keep them healthy. Larger species in particular can really be a handful sometimes, especially if you aren’t used to them.
Iguanas are sometimes shown riding on people’s shoulders. Most newly-acquired iguanas can be very skittish, often fleeing or whipping their tails away from perceived danger. This can be frustrating or even dangerous for someone expecting a docile companion.
Iguanas are sometimes portrayed as being fed live insects (usually flies). Unlike many lizards, iguanas are herbivores. They prefer fresh leafy vegetables to creepy crawly insects.
In various forms of media, chameleons are shown changing color almost instantly to pretty much any color/pattern the background happens to be. This causes chameleon owners to want to see said color changing for themselves. While chameleons can change color, it's actually to indicate mood, health, body temperature, and for territorial displays rather than for blending in (there is one species that does change color to blend in, but it isn't kept as a pet). In fact, it's actually a very bad idea to force a chameleon to change color since it can actually stress out the reptile and make it very ill.
Snakes:
Shows/movies/etc. in general tend to depict a pet snake being fed live food. Although it's debatable whether or not it's humane to feed a snake live food, it is generally considered more dangerous to the snake. This is because a live rat, mouse, or even rabbit will still struggle to escape and can cause injury to the snake. It's generally recommended to feed snakes frozen food (thawed to body temperature) instead. Live food can be dangerous for the owner, also. Snakes presented with live food will likely be considerably more aggressive and more likely to bite for some time afterward, due to having its hunting instincts triggered.
A common sight in media is to see a rather bloated-looking snake due to it eating a large meal. In reality, snakes do sometimes die because their stomachs rupture from eating too large of a meal. Unfortunately, novice snake-owners tend to not know this and think their snake only needs one very large meal a month to sustain itself. Yes, snakes can go for a long time without food (especially after a good-sized meal). But, it's safer to just feed the snake a modest meal more often (how big and how often depends on the size and age of the snake).
One persistent image is that of the beautiful belly dancer with a constrictor (either a boa or large python) draped over her shoulders as she performs. This generally isn't recommended for novice snake owners, especially if the snake isn't used to being handled yet, for a few reasons. For one thing, if the snake has eaten, it may do what snakes generally do when a full snake feels threatened (or is moving around too much). It will regurgitate. Second, constrictors are incredibly powerful creatures. If one feels like it's going to fall off, will tighten its grip. This can cause severe injury to the owner (and, in rare cases, even death). Finally, remember how it was mentioned before that snakes regurgitate if they feel threatened? Well, they do something else to deter predators. Let's just say dancing with a python is far less sexy when it's frightened and decides to poop all over you.
Another misconception is the idea that since snakes swallow food whole, they don't have any teeth. Or, alternately, only venomous snakes have teeth and non-venomous ones don't. In reality, nearly all snakes (with the exception of Egg-Eating snakes) have teeth. Said teeth are small, needle like, and tend to curve towards the back of the mouth. This is to ensure the snake can hold tightly onto prey while swallowing. Combine that with powerful jaws and many first-time snake owners quickly learn just how painful a snake bite can be.
People trying to make their pet Hognose Snake play dead. While this can be fascinating to watch, it is a very bad idea to make a pet snake do this. This is because playing dead is a defense mechanism used to deter predators. It can cause stress in the snake and greatly shorten their lifespan.
On the subject of Hognose Snakes, the idea that they are dangerously venomous has become quite prevalent around the internet. Hognose snakes are rear-fanged; their fangs are located far in the back of their mouths, so in order to bite a human they would have to somehow swallow their entire hand. Even then, their venom is relatively weak. If one bites a human, the worst that will happen is usually prolonged itching and irritation, or at worst some localized swelling.
Novice snake owners generally don't realize they need to make sure the snake is in a locked (or at least very tightly closed) enclosure. Snakes are known to be professional escape artists, so it's important to make sure they are kept securely in their habitats. And, if a snake cannot get out through brute force, it will find a way out by exploring its home and finding any openings large enough to crawl through.
Many people assume that it's ok to be loud around snakes. This is due to the common misconception that snakes are deaf. In reality, snakes can hear, but since they lack ear holes they pick up noise by sensing vibrations in the air or through the ground through their jaws which are then carried to the inner ear inside the head. Loud noises can actually cause severe stress in snakes. Imagine how annoying it is when speakers play really loud music (to the point where you can feel the vibrations coming from said speakers), and what it must be like for an animal that "hears" by feeling vibrations.
Don't think a turtle or tortoise can escape? Try again. Novice turtle owners may let their new shelled friends roam in their backyard only for said turtle to make a getaway. They may be slow, but they are persistent. It's important to keep a close eye on a turtle if it's allowed to spend some time outside lest it manages to make it to the neighbor's yard or into any sort of danger.
Amphibians:
Many shows/cartoons/etc. feature a child (often a male) showing off his (or sometimes her) pet amphibian (often a frog, but newts/salamanders and toads are sometimes featured) by carrying it around or by it poking its head out of the child's pocket. In reality, amphibians have very sensitive skin and it's generally recommended that they are NOT handled unless you have rinsed your hands of any soaps or whatnot that could potentially injure said amphibian.
On the subject of amphibians, several novice Axolotl (a fully-aquatic salamander native to Mexico) owners tend to keep several in an aquarium at once. Axolotls are cannibalistic and, unless they are roughly the same size, will attempt to eat one another.
Novice owners also tend to put gravel (or decorative rocks in general) at the bottom of an Axolotl tank. Unfortunately, since Axolotls feed by sucking water/food into their mouths and swallowing them whole, they tend to accidentally swallow said stones. This can cause severe digestion problems and even death. It's recommended to use either fine sand or simply no covering at all on the bottom instead.
Many shows have depicted a small child's pet frog accidentally slipping out of its owners hands and landing safely on the ground. While some frogs can safely land on the ground from a certain distance, dropping a frog in real life usually results in something similar to what would happen if you were to drop a water balloon, only messier.
Fish
Many novice fish owners will keep several different species of fish in the same tank. Now, while this may be fine with some species of fish, many fish do not thrive well with others. These can range from fish that are simply too aggressive to be kept with other species, to them not having the same tank requirements. Alas, expect a lot of shows/movies/etc. to show a standard-size fish tank with multiple species that would most likely kill one another in Real Life. Sometimes, this may be Played for Laughs wherein the fish behave exactly as you would intend for them to in this situation.
Many forms of fiction depict people feeding their fish standard "fish flakes" or "fish pellets". In reality, many species of pet fish require a specialized diet (see below for a specific example). Not only that, but novice fish owners tend to think they only need to feed their fish a large meal once a day. This can actually cause severe digestion problems in certain species of fish. It's recommended that owners feed their fish small meals at least 2-3 times a day.
Many depictions of fish tanks generally show them containing nothing but gravel (or sand), water, and the fish. In reality, it's best to add small plants (fake or living) and/or decorations that allow the fish to explore and hide. This is for three reasons. First, fish do get bored. Without an area for them to explore, they will end up doing what is known as "glass surfing" in which they will repeatedly slide up and down the same side of the tank. Second, fish get stressed out if they don't have an area where they can hide. Many pet species of fish tend to prefer areas where there are plenty of places to hide from predators, much like how they would live in the wild. And, finally, adding decorations helps more territorial fish feel like they have their own space. They are less likely to attack one another than they would in a non-decorated tank.
Take note: goldfish are cold-water fish. While they're hardy enough to survive in a tropical tank, they won't be especially healthy or live as long as they otherwise might. Additionally, they're very messy, more so than many small tropical fish, and produce so much ammonia that it can overwhelm and poison more delicate fish.
More specifically, many people feel that it's okay to house algae eaters and goldfish together. This is bad because on one hand, most algae eaters are tropical, while on the other they also have a nasty habit of eating off a goldfish's scales.
Fish with very long or delicate fins (or cheeks, in the case of bubble-eye goldfish) need to have only very soft or rounded decorations to prevent these soft body parts from being injured. If you see a fish with long flowing fins being kept in a tank with pointy rocks, please don't do the same at home. However, if those same long fins have a high degree of control, the risk is considerably lower.
Pet fish in fiction are often portrayed as living in open pools without any lid, or in open outdoor ponds in some cases. What some pet-owners don't realize is that the Jumping Fish trope is not limited to wild fish, and particularly stressful conditions and certain fish species that habitually breach the water (such as arowanas) are a risky combination, so a closed-lid tank is recommended more often than not. Piranhas for example need such a tank not to prevent mooks from falling in, but because they're notorious for jumping out of their aquarium to their deaths when kept as pets.
Pufferfish:
People who own pufferfish tend to want to see them "puff up" like they do in the movies and cartoons. The problem? This causes severe stress to the animal (remember, pufferfish inflate themselves to frighten away predators) which can shorten its lifespan. Not only that, but pufferfish can be seriously injured or even die while puffing up. Some species of puffers (typically saltwater species) will even release toxins into the water when frightened. In a small tank, this can result in the puffer itself ending up being lethally poisoned.
Many novice pufferfish owners tend to feed their pets other fish. While puffers will occasionally eat other fish, their primary diet consists mainly of clams, snails, crabs, shrimp, and other shellfish. In fact, it's recommended that pufferfish owners primarily feed their pufferfish shellfish in order to keep the puffer's beak healthy (a pufferfish's teeth are formed into a beak-like shape that grows throughout its life. Without shellfish to wear said beak down, the pufferfish would eventually starve).
Keeping more than one pufferfish in a tank is also a common sight in pet stores (and in fiction). Although juvenile puffers may sometimes swim in groups (often as a way to keep safe from predators), adult puffers are solitary (With the exception of breeding pairs) and are aggressive towards one another. Unless you're trying to get Mr. and Mrs. Pufferfish to breed, or if you have a large enough tank, it's best not to keep more than one puffer in a tank.
Often, novice pufferfish owners will keep Green Spotted Puffers (the most commonly kept pet puffer) in freshwater and often with goldfish. There are two major reasons why this is a bad idea. First of all, GSP are tropical fish. They prefer warmer water compared to goldfish. Second, and more importantly, goldfish are freshwater fish. GSP, on the other hand, are brackish water fish. This means that GSP live in saltier water than goldfish. While GSP can live in fresh water, it greatly reduces their lifespan by several years. They are not compatible with one another.
Likewise, some pet owners (and some pet stores) tend to put GSP with the similar-looking Figure "8" Puffer. While juveniles of both species can live together for a while, they are not suited for being long-term tank mates. For one thing, like with the goldfish above, Figure "8"'s are freshwater puffers whereas GSP are brackish water puffers. Also, GSP grow to be larger than Figure "8" puffers and will attack them out of territorial instinct.
A common sight in various forms of media and in various fish tanks in Real Life is to see a puffer sharing its home with other species of fish. Now, while puffers may tolerate certain other species of fish, they are actually notoriously territorial. Even the small ones are known to viciously attack other fish to the point where there have been accounts from fish owners of their puffer (or puffers) actually attacking and killing fish much larger than itself.
Contrary to popular belief, puffing up isn't the only way these fish defend themselves. Many novice puffer owners learn the hard way that their new fish is more than willing to express its annoyance (or fear) of its owner by biting their fingers. This can actually cause serious injuries due to puffers having extremely sharp teeth and powerful jaws. Larger species have even been known to bite off the fingers of their owners.
A common misconception that both novice and even experienced fish owners make is the idea that fish "breathe water". This is entirely false. Fish actually process oxygen that's dissolved in water through their gills. If they are unable to do so, or if the water is poor in oxygen, the fish will essentially drown.
Invertebrates
While watermelon is a healthy, refreshing summer treat for most animals, one of the exceptions is ironically the ones that are stereotyped as loving watermelon (in Japan): Stag and Rhino beetles. While it's not the worst thing you could give them, beetles have trouble digesting watermelon and it can give them diarrhea (since they can't sweat or urinate, this is the only way for them to get rid of excess water). Apples (that have excess juices squeezed out), bananas, and specially made beetle jelly are a better choice to feed captive herbivorous beetles.
— also known as a moss ball.