Ive heard stories about people taking in bobcat kittens before realizing. But the story that takes the cake is someone raising a dog for years before realizing it was a fucking bear.

https://abc11.com/pets-animals/woman-unknowingly-rescues-bobcat-thinking-it-was-kitten/5570293/

https://people.com/pets/pet-dog-is-bear-not-tibetan-mastiff/