Princess Tutu, Sekai Meisaku Douwa: Hakuchou no Mizuumi Recommendations
Both are inspired in the old classic story of Swan Lake.
Both are about girls who can transform between duck/swan and humanform. Hakuchou no Mizuumi is all about the story from the ballet 'Swan Lake', Princess Tutu is about some more fairytales but mostly Swan Lake and about ballet. Both feature the classical score from Tsjaikovski as well.