Princess Tutu(2002)

Princess Tutu

13-year-old ballet student Ahiru is clumsy, good-hearted and sweet... and has a big secret. The mysterious Drosselmayer morphed a young duck into a girl to give her a mission: help a Prince to get the parts of his heart back. With that in mind, she morphs into Princess Tutu, whose magical dances ease the pain and purifies the bad feelings.

(Source: Anime News Network)