Samantha of Bewitched turned a horse into a woman just to understand what the horse wanted to say. The changeling was not happy with this: "I've got nothing to swat flies with!"
She did this in another episode as well, turning a cat into a woman to model for her husband's boss. In this case, though, the cat-turned-woman actually likes being human.
According to the Shanshu Prophecy, a vampire with a soul will earn the right to become human again after playing a pivotal role in the Apocalypse. So far, Angel and Spike are candidates.
Illyria started out life as a routine Sealed Evil in a Can, escaping from her tomb in a sarcophagus and possessing the body of Winifred "Fred" Burkle, a series regular. Illyria, having shrank down from the 50-foot Cthulhu into a skinny (albeit indestructible) corpse, is forced to throw in her lot with Angel's crew, being stuck with a form which would serve as catnip to her demonic cohorts. Adding to her problems, remnants of Fred's brain patterns start affecting the creature's personality, causing it to unconsciously reconstruct her host's old life — including bonding with Fred's grieving lover, Wesley. By the series finale, it's almost impossible to tell where Fred ends and Illyria begins.
In the "I Will Remember You" episode of Angel, the titular vampire is made human by accidentally mixing his blood with the blood of a Mohra demon with regenerative powers. After becoming a Sense Freak, noting that he doesn't like yoghurt, having mucho bed-time with Buffy (in a Contrived CoincidenceCrossover) and generally enjoying the sun he employs an in-universe Reset Button when he learns that his humanity will prevent him from fighting in the upcoming apocalypse and thus cause the deaths of many people, including the Slayer. His reward and punishment (two for one!) was being the only person to remember the lost day.
Anya from Buffy is another example. Anya was originally a thousand-year-old vengeance demon who took human form to curse unfaithful men. Then in "The Wish" she loses her powers, and becomes human. She eventually resigns herself to being human. Her unfamiliarity with human social conventions (particularly not talking about sex in public) is a running gag/theme.
Or so it seems, at first; it actually turns out she originally was a human, born in Sweden during Viking times. And she was just as weird back then, while other vengeance demons like Halfryk don't seem to have any such foibles.
Also from Buffyverse, Darla, the first vampire seen in Buffy the Vampire Slayer, is resurrected as a human complete with incurable disease that nearly killed her originally but with the full memory of having been a vampire intact. Later, she is turned into a vampire again, but suffers from bouts of human emotions when she becomes pregnant with Connor.
In Charmed (1998), a snake, a rabbit, and a pig were turned into humans by a group of girls who wanted a date for a dance. The spell was temporary but the transformed men wanted to remain human permanently.
The Dinosaurs episode "Little Boy Boo" has Robbie telling Baby a scary story about a "wereman", starring himself as the victim of the curse.
"Daleks in Manhattan"/"Evolution of the Daleks" has a halfway example: while stranded in the 1930s, Dalek Sec absorbs a human ally and becomes the first Human-Dalek. Admittedly, he doesn't really look like a human except for having arms and legs — he still has Dalek tentacles and the single eye on his new head — but his worldly perspective becomes more and more human during the rest of the arc, to the point that he begins to consider the Daleks' creator Davros to have been wrong and wants to help and be helped by the Daleks' sworn enemy, the Doctor himself. Unfortunately, this ends up getting him killed by the other Daleks as a heretic, possibly sacrificing himself to save the Doctor from the others.
"Human Nature"/"The Family of Blood" reveals that Time Lords can use a device called a Chameleon Arch to turn themselves human (or any other Human Alien species) and put their memories and Time Lord nature in a fob watch. Opening the watch turns them back. The difference from most examples of this trope is that Time Lords, while transformed, have no real memory of their former selves. The Doctor, as a human, has strange dreams of his Time Lord life, but doesn't doubt his humanity and seems perfectly content.
"Utopia" reveals another Chameleon Arch user: The Master, a.k.a. "Professor Yana". As Yana, the Master hears the sound of drums that he started hearing when he looked into the Time Vortex as a child, but just like the Doctor earlier, doesn't suspect the truth.
"The Doctor's Wife": The TARDIS' soul is ripped out and stuffed into a human body.
"Fugitive of the Judoon": Ruth Clayton turns out to be a Time Lord hidden via a Chameleon Arch — and not just any Time Lord: she's a previously unknown incarnation of the Doctor whose positioning in their overall timeline is a mystery, since she and Thirteen don't have any memories of ever being each other. Her husband Lee is also implied to be a Time Lord or other Gallifreyan turned human as well, but with his memories intact.
After Adalind attacks Nick, his family, and his friends several times, Nick tricks her into biting him, which forces some of his Grimm blood into her system, stripping her of her hexenbiest powers. She spends the rest of the season working to recover her powers, feeling that she is nothing without her magical powers.
In retaliation, Adalind takes a potion that transforms her to look like Nick's fiancee, Juliette. Nick and Adalind-as-Juliette then have sex, which strips Nick of his Grimm powers. Nick, who at one point hated being a Grimm, is desperate to recover his powers, realizing how much he's come to rely on them to protect the people around him.
In Hercules: The Legendary Journeys a pig named Katharine fell in love with Hercules from a previous adventure where he got turned into a pig. Aphrodite granted her wish to be human, and as a bonus, gave her a body identical to hers (they were played by the same actress). However, while Hercules treated her well, she ultimately could not adjust to living like a human and asked to be changed back. The final straw for her was that her barnyard friends didn't recognize her and they couldn't understand each other.
In I Dream of Jeannie, Jeannie turned a chimpanzee named Sam into a man. Sam winds up liking being human.
The Journey of Flower: Tang Bao, a spirit worm, turns into a human in episode thirteen.
Mako Mermaids: An H₂O Adventure has a trio of mermaids banished from their pod when they accidentally allow a human boy to enter their sacred moon pool and become a merman. To get back, the trio use magic to take human appearance and have a hard time at first from walking to figuring out what to eat and human customs.
The trio soon discover the school principal is a mermaid herself who made herself appear human years before after falling in love with a human boy and she helps them get used to their new forms.
Rita's cat Poseidon winds up as a copy of Evie after spilling some potions and seeing Evie in the hallway. Though apparently instantly able to speak, Poseidon gives no acknowledgment whatsoever that a change even took place. However, s/he still behaves like a cat, knows that the mermaids are mermaids (which, at this point, the real Evie doesn't), and doesn't know things Evie knows, causing no end of confusion for the main cast. (Especially for Evie, who is told multiple times that she was just seen somewhere she knows she wasn't.)
It's later revealed that Zack was born as a merman; his mother transformed him into a human using a powerful spell that couldn't be broken by contact with water until he fell into the Moon Pool.
Fairy tale characters such as Gus (the mouse from Cinderella) and Ariel the mermaid appear in Once Upon a Time, where they transform into humans from their respective species.
In Once Upon a Time in Wonderland, Cyrus begins the series as a genie (once a human) but later becomes a human. Additionally, Will briefly becomes a genie but is restored to a human again.
The Power Rangers Turbo episode "Cassie's Best Friend" has Cassie's dog Jetson become a human being.
Black cat Luna of the live-action Pretty Guardian Sailor Moon has the ability to turn into a human girl. Unlike prior incarnations in the franchise she turns into a small child, and even a Sailor Senshi. However, she still acts like a cat sometimes, and sneezing changes her back.
The Red Dwarf episode "D.N.A." has Kryten turn human, much to his surprise and pleasure — until he finds out that his spare parts (most notably his spare heads) hate him for it. Just take a look at the scene. And the photos were an example of Enforced Method Acting. And his eyes no longer have a zoom mode. And his nipples no longer pull in radio signals. And the whole last-chicken-in-the-shop look for certain body parts.
The Nickelodeon TV movie Rufus is about a dog who becomes a teenage boy by wearing a magical necklace. Like the above, it's filled with My Instincts Are Showing moments. The sequel has Rufus falling in love with a girl who owns a similar necklace — and she's really a cat!
This is also a punishment for Q in the Star Trek: The Next Generation episode "Déjà Q". Having less than a microsecond to decide, he chose to be human in the hopes that Picard and crew would protect him. Then spent most of the episode lamenting that he should have asked to be turned into anything else other than human.
Star Trek: Strange New Worlds episode "Charades" has Spock become fully human thanks to extradimensional aliens accidentally "repairing" him by using Christine Chapel as the template. While M'Benga and Chapel are working on a cure, Spock has to deal with his fiancée T'Pring insisting on a ritualistic tea ceremony involving the couple and their respective parents (minus Sarek, who's still not on speaking terms with Spock). Since Spock's future mother-in-law T'Pril is disdainful of humans, Amanda has to teach Spock how to pretend to be Vulcan to fool his future in-laws. He not only succeeds but also gains an insight into just how much his mother has suffered for his sake.
Seven of Nine started out as human, became a Borg as a kid, and was forcibly brought back down to human (more or less) by the crew of Voyager. While initially not happy about it (to say the least), Captain Janeway guided her through the process of rediscovering her humanity through time, patience, care, Les Yay...
Also, it was impossible to communicate with Species 8472 before they started taking on human form, and afterwards we never saw them in their tripedal, purple-skinned, cross-pupilled Supernatural Gold Eyes form again.
Castiel in the episode "Two Minutes to Midnight". After his previous altercation with other angels in "Point of No Return", Castiel turns up in a hospital and finds that he has become pretty much human. He shows that there's just enough angel left inside him to resist Pestilence's powers.
As of the end of Season 8, not only has Castiel been subjected to this again, but so has every angel in Heaven, barring the one vengeful angel who forced them all to fall.
In the final season, Jack turns Chuck, the character who serves as the in-universe equivalent of God into a human so that the latter can understand the difficulties of being a human, particularly trying to survive in a universe where the omnipotent deity treats your life like a cringey reality TV show. Chuck is appalled realizing that he is no longer in control and that he will eventually die, just like all the other mortals.
The 1997 TV movie Whiskers is about a boy who spends too much time with his cat. His cat was turned into a human with the help of an Egyptian goddess wishing he has a human friend. Filled with My Instincts Are Showing moments.
Used in Wizards of Waverly Place when Alex gets Justin to turn a couple of guinea pigs into humans for some information.
The X-Files: The is the twist at the end of the episode "Mulder and Scully Meet the Were-Monster". The Monster of the Week is not, as Mulder suspects, a man who turns into a reptile-monster, but a reptile-monster who turns into a human, as the result of being bitten by one. Even Mulder doesn't know what to make of that one.
at the scene. And the photos were an example of Enforced Method Acting. And his eyes no longer have a zoom mode. And his nipples no longer pull in radio signals. And the whole last-chicken-in-the-shop look for certain body parts.