Lilly's Transformation(aka Crookshanks was actually Lilly Potter)

Lilly spends twelve years as a cat, until Hermione buys her at the Magical e then goes to Hogwarts and convinces Severus that she's human and he figures out how to transfigure her back. While she was a cat, she was owned by a member of the Pureblood elite and learned their customs,as well as powerful magic. She kills Voldemort teaches magical etiquitte and falls in love