Kyo from Fruits Basket:
Just because he was born the cat, he was looked down upon from others... his father went berserk and his mother killed herself... Everyone made fun of him and he had no "true friends" throughout his childhood. Also he had to deal with the cat's true form.
Momijji
He was born the rabbit and his mother couldn't take it.... She chose to forget all about him! That was so sad... he had to deal with knowing that his mother hated him and his very existence....
Lots of people from fruits basket had a bad past....