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Human Alien Discovery if Raised by Humans. Also have a look at Nature Versus Nurture.
Seconding Nature Versus Nurture, with subtrope Nurture over Nature when this is extremely successful. Elf is a good example of both the extreme form where the adopted takes on not just cultural but also physical traits of their adopters, and the common handwave of growing up around magic did it.
You can also get into variations of Raised by Orcs and Orc Raised by Elves when there are problematic conflicts between nature and nurture. Say, in a World of Funny Animals, the wolf adopted by sheep is a kindhearted vegetarian, but has occasional cravings for (non-sapient) meat that the story treats like a parody of Horror Hunger, complete with the family making Supporting the Monster Loved One-style secret trips to the burger joint across town. A dramady plot point has the Nosy Neighbor catching Dad Sheep hiding a bacon cheeseburger in a Salad King bag to carry it inside, after which Wolfie notices several of her long-time clients suddenly hiring other babysitters.
Interspecies Adoption is in play. A character is raised by another species at a young age. However, as the years go by, the adoptee starts to develop traits associated with the species that they were raised by, traits the adoptee's species normally doesn't have. For example, Elf is about a human who is raised by Christmas elves. While the human, Buddy, has a lot of the traits that most humans have, like his height and the way that he ages, he's also developed traits that elves mainly have, like how he seems relatively healthy despite only eating junk food for most of his life, and is an exceptionally skilled toy by human standards. Is there a trope for this? Thank you!