The Fairly OddParents (2001)

Have you ever wondered what makes Vicky so mean? The answer is surprisingly easy: She has a bug up her butt.When timmy wishes that his mean and nasty babysitter would be nice, a magical bug containing Vicky's evil essence crawls crawls from out of her butt leaving behind a sweet and docile teenager. Vicky is now a total saint. But the evil Has to go somewhere, which means that the bug is on the hunt for its next host.