Unfortunately, that's where coral snakes are from. It only works because she was bought in a pet store, not found somewhere. And being a magical version of the coral snake, it was logical that she'd be in the Magical Menagerie. As Sev pointed out, her venom is a useful potions ingrediant... There are only two native snakes in Britain, and it's illegal to make pets of them. IIRC, they're also non-venomous. I wanted to give Harry a snake that was 1) pretty 2) could grow to be pretty big and 3) was poisonous. Miss Sarah Sue Rae is really there to serve as Harry's confidante--he doesn't trust grown-ups, and she's a very non-threatening person for him to talk to. It's going to take along time to break through Harry's walls. Severus is trying, and it's starting to sink in that Harry is his complete responsibility. He figures that he's already up on the Dursleys by providing Harry with medical care and adequate clothing.