Babylon 5: In the short story "True Seeker" a young Narn girl born in 2353 is evacuated from Narn. Adopted by human parents she was raised on Earth with the name Jerrica Thomas but was sent to the Narn Academy in San Francisco in order to learn about her people. Returning to Narn in 2269 she discovered that her father was the religious figure G'Kar.
Bravelands: After his father is killed by a rogue lion, Fearless the lion cub ends up being adopted by a troop of baboons. He bonds with several baboons like Mud and fellow protagonist Thorn, but he also faces prejudice from other baboons for being a larger predator. He ends up kicked out of the troop at one year old because the new leader doesn't like him.
In Brimstone Angels, heroine Farideh and her twin sister Havilar are tieflingsnote humanoids with distant fiendish ancestry, notable by their horns, tails, and Monochromatic Eyes who were adopted and raised by a dragonborn named Mehen.
Bunnicula has a rare example of the child "adopting" the parent: Howie, a wire-haired dachshund puppy, inexplicably latches onto Chester, a cat, as his "Pop" when he is adopted by the Monroe family following the second book (as opposed to Harold, the actual older dog among the Monroe pets, who Howie calls "Uncle Harold"). Chester, who doesn't exactly hold dogs in high esteem, has no idea why the kid formed this attachment, but eventually just resigns himself to it.
The Call of the Swamp by Davide Cali and Marco Soma tells the story of an anthropomorphic axolotl found on the shore of a swamp and taken in by a human husband and wife who are infertile. He has a fairly normal humanlike childhood, with some accommodations for his amphibian biology like sleeping in a bathtub rather than on a bed. But one day he catches a faint scent of his old swamp home and decides to go back there to figure out more about his heritage. He gets along with the other swamp inhabitants and is happy to be able to explore an environment where his axolotl biology thrives, but he ultimately decides that he'll be happier with his human parents.
The picture book A Child Is a Child by Brigitte Weninger is about two orphaned frogs. They are discovered by a mole, a blackbird and a hedgehog who all agree that they don't have what it takes to take care of them. Then Mama Mouse comes along with her children and decides to adopt the frogs. Mr. Hedgehog tells her that it's not that easy, but she replies "That's ridiculous! It's simple. A child is a child. All children need a place to live and play, good food to eat and someone who loves them!" She then enlists of all their help in raising the frogs.
Child of the Wolves is about a husky puppy named Granite who is adopted by wolves after he escapes his new owner.
Cradle Series: Lindon briefly meets a black dragon who is outspoken againstthe way dragons treat humans, and has even adopted a number of humans himself. Of course, Lindon only finds this out after he's nearly killed the poor bastard and used his Cannibalism Superpower to drain most of his power (and some of his memories, which is how he found out). Lindon quickly assures himself that he'll recover.
This comes up in The Demon's Lexicon when Nick is revealed to be a demon, raised mostly by his brother Alan.
DFZ: Legally, Opal is the adopted human daughter of Yong, Great Dragon of Korea. This is complicated a bit because she is actually the daughter of Yong's human consort, genetically engineered under Yong's orders and always intended to be his daughter. While they are not actually biologically related (since Half Human Hybrids aren't a thing), magically is another story. Between the dragon ability to take unrelated dragons into their clan, and the human ability to subconsciously shove magic at anything they consider important, they are father and daughter in every way that matters. Much of the plot of the third book revolves around them taking advantage of this connection to combine human and dragon magic in ways that would normally be impossible.
Dinosaur from Dinosaur Vs is a young dinosaur adopted by a human couple.
In Guards! Guards!, Carrot is a human adopted by dwarves. He is completely oblivious to this, even when his adoptive father tries to explain that there's a reason he was always too tall to fit in dwarf passages correctly. Notable in that even after it's been explained to him and he's accepted that he is biologically a human, Carrot still considers himself a dwarf, albeit a very tall one. Though, in Discworld being 'a dwarf' is as much a matter of cultural identity than it is a fact of one's species.
In Snuff, Miss Felicity Beadle's mother was raised by goblins. Since the humans in the Shires had a severe case of Fantastic Racism, they interpreted this as a child being kidnapped by goblins, and "rescued" her.
It's mentioned in Raising Steam that it's very hard for a dwarf/human couple to have children, but they frequently adopt. Presumably, the child would be this to at least one parent.
One Doctor Who short story said that the Doctor's granddaughter, Susan, and her husband, David, adopted three orphaned human children, whom they named Ian, Barbara and David Junior. (David Senior may be human, but he sure as hell didn't marry one.)
Bella's mother and siblings were taken by humans when she was a month or two old. Bella was raised by a feral, nursing cat she calls "Mother Cat". When Bella refers to her mother, she's always talking about her adopted mother. Even after Bella leaves Mother Cat and grows into an adult, Mother Cat still recognizes her.
While Bella is trying to get back to her owner, she sees poachers kill a cougar mother. The cub starts following Bella around. Bella dubs her "Big Kitten". Big Kitten ends up vital to Bella's survival as she has better hunting instincts than Bella. The two end up separated when Bella gets taken in by humans but reunite again months later. Bella comes to the realization that Big Kitten can't live with her and her owner, but Bella wants too badly to be with her human that she can't stay with Big Kitten in the wilderness either. The two have to part ways in the end.
In the short story "Homecalling" by Judith Merril a preteen girl and her baby brother survive a spaceship wreck that kills both of their parents. They are found by an insect-like alienHive Queen that adopts them and does its best to take care of them despite being completely unfamiliar with humans.
James and the Giant Peach: Less so in the original novel, but in the movie and the musical adaptations, we have the human child James, an orphan, who is adopted by his humanized insect companions. They turn out to be very loving, adoring parent-figures to James, unlike his abusive guardian aunts who he was living with after his mother and father died.
In The Kitten Who Thought He Was a Mouse, the titular kitten is abandoned by his family before he even opens his eyes, and is taken in by a family of mice, growing up to believe that he is a mouse himself. Eventually, he discovers his true species, but remains in contact with his mouse family.
Kitty Cat Kill Sat: Lily ad-Alice was once an ordinary housecat adopted by a human woman. This was not just a polite way of describing a pet/owner relationship; Alice gave Lily her bond name, explicitly referred to her as her daughter, and the reason that Lily is now immortal is because when Alice's co-worker tried to do something that involved killing Lily to activate the machine, Alice shot him and died herself. Centuries later, Lily still refers to Alice as her mother, even though she's long since become an Uplifted Animal.
The Last Human: The titular last human is known as Sarya the Daughter, adopted by Shenya the Widow. It should be noted that Widows are a Proud Warrior Race who practice The Spartan Way on their own hatchlings, much less adopted children. "The Daughter" is not a description or an honorific, it is a title that Sarya earned through an eight-day trial that, statistically, would most likely have resulted in her death.
When Electra was a baby, merpeople found her floating alone in a lifeboat after a big storm. She was given sea powder, which lets her breathe underwater, and was adopted by the mermaid Maris. Electra likes living in the ocean and thinks of herself as a mermaid, even though she's the only merperson with legs.
It's also common for mer families to take in orphaned dolphins until they're old enough to travel alone. In Mermaid Island, Maris adopts a bottlenose dolphin named Splash whose parents were killed by sharks. Splash and Electra become best friends.
RuneScape: Betrayal at Falador, Kara was adopted and raised by dwarfs.
The Sheep-Pig, in which a piglet is raised by a sheep-dog.
The Cuckoo Child, in which an ostrich is raised by a pair of geese (although for some time the ostrich was assumed by his parents to just be a strange goose, even if the farm that owned them knew what had happened).
Dragon Boy, in which an orphaned boy is taken in by a pair of dragons, uses the trope in a more fantastic way.
Magnus Powermouse has a lesser version of this; Magnus is permitted to consider Roland the rabbit his uncle, although his parents are both alive, as Roland expresses a fondness for children and imagines himself to have nieces and nephews from his long-absent siblings.
Felicity the duck is either this or Intergenerational Friendship with Daggie Dogfoot the pig in Pigs Might Fly as Daggie's mother is still around but Felicity is the one who teaches Daggie how to swim.
In Spectrum by Sergey Lukyanenko the human heronote a private investigator searching for a runaway girl, who ended up on the path to discover secrets of long-lost alien civilizations ends up adopting a teenage alien bird-girl as a "reward" for saving her life. His Love Interest cracks jokes about an "interplanetary paedophile", but demonstrates that she can be a capable mother, despite being in her late teens herself. The adopted girl turns out to be the Chosen One prophecised to wake her planet from its millennia-long Diesel Punk stasis where rigid laws allow the population not to think. She ends up becoming the queen equivalent and legally adult and staying home.
Galaxy of Fear has Hoole, a Shi'ido Shapeshifter, adopt the human kids Tash and Zak. He's actually their uncle, since his brother married one of their aunts, and Shi'ido culture encourages him to take them in despite barely knowing them.
The X-wing series gives us Viull "Skut" Gorsat, a Yuuzhan Vong abandoned by his biological parents and adopted by a human family. His human dad is the reason he joins Wraith Squadron, because as a child he was told stories of how they rescued his father.
There is also Tahiri Veila, who was adopted by the Sand People.
Han Solo himself also falls into this category. He had been orphaned at a very young age, and while working for a rather nasty thief, he gets taken in and cared for by a kindly, elderly female Wookiee. Han even comments in a later novel that due to the love she gave him, and the fact that Chewie and other Wookiees have saved his skin on countless occasions, he feels he owes the Wookiees a life debt, not the other way around.
In the original series novel "Child of Two Worlds" a Cyprian girl named Elzura is adoped by the Klingon General Krunn after being taken in raid that had killed her father. Given the Klingon name Merata she comes to consider herself a Klingon. Ultimately her family realizes that Merata could never adapt to living on Cypria after living as a Klingon for so long, and Spock helps Merata escape back to the Klingon fleet.
Star Trek: The Next Generation: In the novel Reunion identical twin sisters Gerda and Idun Asmund were adopted by the Klingon Warrokh after the Alpha Zion colony was destroyed by Klingon raiders and the girls were the only surviors.
Star Trek: Deep Space Nine: In the relaunch novel "The Never Ending Sacrifice" Captain Sisko's decision to return Rugal Pa'Dar to his adopted family is overturned on the grounds that the decision had not followed Bajoran law, making him the legally adopted son of the Bajorans Proka Migdal and Proka Etra, and by extension a Federation citizen. Rugal in turns adopts the human orphan Hulya Kiliç, who he had encountered at the ruins of a farm on Ithic II that Rugal's girlfriend Penelya Khevet had previously owned.
The eponymous fruit bat of Stellaluna gets separated from her mother by an owl. She gets taken in by a family of birds, where the mother makes her eat and sleep like them instead of like a fruit bat. Stellaluna eventually reunites with her birth mother and learns proper fruit bat habits but still considers the bird siblings to be her family.
In Team Human, Kit is a teenager raised by a family of vampires; he's spent very little time around non-vampire humans.
The children's book Tyranosaurus Drip by Julia Donaldson is about a duckbill dinosaur "adopted" by tyrannosaurs. (Mother Tyranosaur can't count, and doesn't realise there's one more egg than there should be.)
"The Ugly Duckling" is a classic example of this trope. A swan raised by a duck.
In the Warrior Cats novella Hollyleaf's Story, Hollyleaf attempts this for a day or so with a fox cub lost in the tunnel, finding it and caring for it before later bringing it back outside. She encounters it a year later and happily greets it, only to find that it does not remember her and it attacks her.
In the 1920s Winnie the Pooh books, Tigger (a Tiger toy) lives with Kanga and Roo (Kangaroo toys) and is considered part of the family.
The children's picture book Wolfie the Bunny is about a baby wolf left as a Doorstop Baby for a rabbit family. The older rabbit sister is insistent that Wolfie is going to eat them all someday, especially when Wolfie grows to be much bigger than her, but an incident with a hungry bear manages to awaken her Big Sister Instinct.
◊. Due to Buru-Buru's actual mother passing away after giving birth to him early in the story and Muku-Muku losing her own child moments after birth. While not mentioned, Buru-Buru's father
