Aesop'sThe Cat-Maiden has a cat transfigured into a woman to settle a bet between Jupiter and Venus. Jupiter thinks that instinct and nature can be transcended and Venus says that "nature will out". They turn the cat into a woman and give her to a man to wed. Jupiter thinks the bet is won until Venus releases a mouse into the wedding hall and the maiden pounces on it, revealing her true nature.
In his collection of Aesop retellings, Aesopus Emendatus, Ambrose Bierce subverts this by having her react like a woman— in such an over-the-top Eek, a Mouse!! way that her potential suitor is irritated.
Animorphs: There are two examples of animals gaining morphing power from touching the morphing cube. One, an otherwise incredibly dangerous Cape Buffalo, nearly achieves sapience but then gets killed off. The other is an ant that partially morphs into Cassie.
Ax counts too. He morphs human and then is really bad at it—he plays with his 'mouth sounds' and goes berserk around food, and he hates morphing human most of the time, always saying humans are unstable on two legs.
Elfangor turned into a human permanently so he could spend the rest of his life on Earth with Loren. Alas, the universe still needed him to be an Andalite warrior. The Ellimist took him away from Earth, and the only evidence that he was ever there at all was his and Loren's son Tobias.
This happens at the end of several Dinoverse books. The first time, three Pachycephalosaurs briefly possess three human boys and jump around before being sent back home. The second time, in Please Don't Eat The Teacher the fifty-foot Acrocanthosaurus called Green Knight is killed and ends up Sharing a Body with Will Reilly, which appears to be permanent. He's content, though, since he can be with Patience like this. The third time, in Dinosaurs Ate My Homework, the intelligent saurian Alternate Universe version of JD takes his body permanently and alone.
This happens to Nanny Ogg's horrible cat, Greebo. Normally, he's ugly and foul-smelling, but when transformed into a human (in Witches Abroad and Maskerade), it is claimed that "His left eye glittered with the sins of angels, and his smile was the downfall of saints (female ones, anyway)." He has been described as looking evil in an interesting sort of way, like a pirate who really understands the term of "Jolly Roger", or a romantic poet who gave up the opium and tried red meat. In fact, the best brief descriptions are that:
He could swagger while asleep. Greebo could, in fact, commit sexual harassment while sitting very quietly in the next room. Nanny: He looks aristocratic. Granny: He looks like a beautiful, brainless bully. Nanny:Same thing.
This also falls in line with the Disc's general opinion of cats: elegant, beautiful, brainless bastards.
In Witches Abroad, Granny's evil sister Lily also did this to a wolf, partially anthropomorphizing it to fulfill the Big Bad Wolf role in a living fairytale. This is treated as a monstrous act, because predator minds having to think like a human drives them insane and makes it impossible to live as either a wolf or a human. In the end a woodcutter gets called in for a Mercy Kill. (This doesn't apply with Greebo because cats have enough poise to pull anything off.)
In the same book, the Sisters who guard Emberella are actually snakes, and the Prince Charmless she's supposed to marry is a frog. Granny darkly comments that Lily was always good at making friends.
Inverted and later defied by the Librarian, who was a human to begin with before being turned into an orangutan, and has carefully destroyed all evidence of who he was so no-one will get the bright idea of trying to change him back.
Brother Lupine in Reaper Man is a reverse werewolf, who takes the form of a hairy bestial humanoid on the full moon, and the rest of the time is just a rather intelligent wolf.
The dragons in Dragons in Our Midst. Though technically voluntary, there was pretty much no other option they could take, and most if not all of them certainly would have rather stayed dragons.
One of Harry Dresden's allies in Fool Moon, Tera West, is revealed to be a wolfwere, a wolf who can take the form of a human. After the death of her lover, she goes back to the mountains to live with her pack.
In Skin Game, the archangel Uriel temporarily turns himself into a human to heal and empower Michael to enter Hades, knowing that if Michael dies, Uriel will lose his powers and remain mortal. While he's sitting around being a useless human, the Carpenter family makes him useful by having him do chores around the house.
In The Fallen Moon, this is Skade's punishment for killing humans.
In one of the Goosebumps books, a bee's mind is accidentally swapped into the body of a human boy. The bee does not seem very comfortable in a human body—it continually makes buzzing noises and attempts (unsuccessfully) to eat nectar from flowers.
Bulgakov likes to play with it, usually for ironical purposes, like in Heart of a Dog: after a lab experiment on rejuvenation, a friendly dog gets turned into a downright rotten human (thanks to the character of the organs’ dead donor). He gets changed back and forgets his human experience completely.
Incarnations of Immortality: In On a Pale Horse, to grant a client's last request for a good story, The Grim Reaper tells a story about a whale who gets transformed into a woman. She falls in love with a man, but gets disgusted when she finds out he's a whaler. Her lover tries to explain that whale meat is the village's main source of food, but she dumps him and returns to the sea. However, she eventually meets a whale who is really a transformed squid. He is trying to investigate why whales hunt his kind. She tries to explain that squid are the whale's main source of food, then realizes that humans and whales aren't so different. She returns to human form, reconciles with her lover, and they get married, presumably Happily Ever After.
A rather neat example in Geoph Essex's Jackrabbit Messiah: Caleb O'Connor was involuntarily transformed into a human sometime in the past. Interesting in that: a) it happened long before the events of the book (and appears to be permanent); b) the character doesn't seem to mind much (aside from wondering who did it and why) and has adapted very well; and c) the book isn't primarily aboutthat character, so the reader never finds out how it happened either. Before the casual reveal, there are some subtle and not-so-subtle hints, including the character's Meaningful Name (both first and last!), and what Indra calls the character when they meet.
In the first Keys to the Kingdom book, it is mentioned that being made mortal is one of the punishments for disobedient Denizens.
In Alan Dean Foster's Kingdoms of Light the familiars of a fallen wizard are transformed into humans and forced to cross a weird dimension that represents the land of light (in the form of a huge rainbow) in order to bring back the power of light to a now darkly colored world. Needless to say with a snake, two cats, a dog, and a bird all turned human with a sudden realization of human feelings and emotions things get a little... odd.
And in Kockroach, a cockroach inexplicably turns into a human being (reversing Kafka's "Metamorphosis"). Rather than trying to find a way back, he makes good on his predicament, taking advantage of everything that comes his way and thriving, because despite his new body he's still a cockroach.
In the book The Last Unicorn by Peter S. Beagle, the ageless unicorn is turned into a young woman to protect her from being captured and imprisoned with all the other unicorns. She is horrified at first, mostly because the transformation makes her mortal, something she's never experienced before. But the longer she remains transformed the more she forgets her unicorn nature and her quest: at the pivotal moment she begs to be allowed to stay human and marry the human prince she has come to love. The book ends with her a unicorn once more, triumphant and immortal but forever tied to human emotions like love and regret.
Happens to the titular character in The Little Mermaid. It's an extremely painful experience though - every step she takes feels like she's walking on knives - and she's unable to talk in human form, as the price for the transformation was her tongue.
Animals in Magical Girl Raising Project are occasionally chosen to become magical girls, whereupon they gain a human form and intelligence. Their normal forms remain the same however. Examples include Tepeskemei, an Egyptian Tortoise, and Cherna Mouse, a hamster.
The titular character of the Dutch children's novel Minoes by Annie M. G. Schmidt. She was a cat, and one day she woke up as a lady. It was later adapted into a movie starring Carice Van Houten.
Tragically, in Greer Gilman's Moonwise. When Sylvie disappears before Ariane's eyes, Ariane has to find her, hampered by the fact that she has no "mind's eye" — she is incapable of mental imagery note true for the author: "Sadly, I can't see pictures in my head. Which is strange, because I'm fiercely visual — I minored in art history, I design things and I draw (too seldom) — but the theatre of the mind is dark. And that changes everything: not only how I read and write, but memory, my sense of time. ". Going by intuition, she accidentally calls to her and embodies a "lightborn" child. The lightborn are spirits who watch over a particular place, in this case the forests of Cloud, one of the worlds Ariane and Sylvie think they made up (and didn't). The lightborn is devastated at finding herself trapped in human flesh, a grim, stark and painful fate. She rages for hours, and refuses to sleep fearing she'll forget. But she is able to find her way back to Cloud, taking Ariane with her. The story is how they find Sylvie, and the lightborn along with many others (who were turned to stone!) is ultimately returned to her true shape.
A Pelicans New Clothes A Story From The City is about a pelican who turns into a human after putting on clothes. He eventually decides to turn back into a pelican after seeing how miserable being a human is.
This trope is the entire point of the novel Shoebag, in which a cockroach finds himself transformed into a human and wants to transform back. Another character is eventually revealed to be another transformed cockroach, who acclimated to human life and didn't want to return even when they found a way.
In the Tom Holt book Snow White and the Seven Samurai, the big bad wolf is turned into a handsome prince (by way of a frog), and isn't very happy about it.
In Alan Dean Foster's Spellsinger, turtle wizard Clothahump coerces Mudge the otter into assisting Jon-tom by threatening to transform Mudge into a human if he refuses. In Paths of the Perambulator, in the same series, Jon-tom's animal companions are all unwillingly transformed into humans by a surge of wild magic, and he has to play Rick Springfield's Human Touch backwards to change them back. Also played for ironic laughs as Clothahump announces this as the "Worst transformation yet" (With everyone besides Jon-Tom agreeing) despite they had already been turned into giant Crabs, Giant Caterpillers, Switching Genders and turning into literal viruses only to temporarily catch said diseases afterwards.
Tales of Kolmar: The king of the Kantri, Akhor, become human at the end of Song in the Silence. As the one Kantri most fascinated by humans - he actually used to try to walk on two legs like them, but it hurt too much - he's initially very happy about it, enjoying the stronger sense of touch and the much more dexterous hands. There's a point in The Lesser Kindred where he realizes he can't fly, and later when it really sinks in that who he was is gone, which are both marked with grief and sorrow. It's very mixed. But it does mean he can be with his beloved. In Redeeming the Lost, he becomes a Kantri again and mourns his human shape.
In The Immortals quartet, Numair tells Daine that because he turned an enemy mage into a tree, a tree somewhere has been turned into a human. Due to a large amount of fan mail asking about the tree, the author eventually wrote a short story about him, "Elder Brother".
In the Trickster's Duet, it's established that crows can turn into humans if they want, but only one character becomes human permanently. And on the flipside, the raka, the humans of the Copper Isles who are cousins of the crows, have stories about rare members of their people who chose to become a crow.
Treasure in the Heart of the Tanglewood has one of the few true subversions of this trope. Year after year, one Knight in Shining Armor after another rides into the Tanglewood to fight the monster within, and year after year, they never come out again. The beast within turns each knight into an animal, then kills them, and tells the protagonist that these were the true forms of the knights, which they had been shifted out of for use as Cannon Fodder. The subversion is that he lied, and shapeshifted beings in this setting don'tdie as themselves. The knights were human all along.
At the end of A Twisted Tale: A Whole New World, Jafar wishing all magic in the world away has the side effect of turning Genie into a human, with the only hint to his former nature being a slight blue tint to his skin. Since magic is as natural to Djinns as something like walking upright or reading books is to humans, he's not happy about this turn of events. And he's none too fond of the whole walking thing either.
When the Angels Left the Old Country: After Uriel the angel gets a name, it starts becoming more and more human, having to breathe and eat when it didn't before.
Wicked: Princess Nastoya is a sapient Elephant who has been put under a spell that turns her into a human. Elephants are hunted by many, but the Scrow tribe reveres them so they don't harm her.
In Worlds of Power, Dr. Light accidentally turns Mega Man into a human while running him through a duplicating machine.
