What about jellyfish? There are very few TFs of those...
Or maybe trilobite, sea-sponge, coral polyp, sea anemone, hagfish, lamprey, or- wait, here's a great one: the Tiktaalik, an early land animal (
http://en.wikipedia.org/wiki/Image:Tiktaalik_BW.jpg ). It looks kind of cute, in a way...
Or you could do a mammal that nobody's every tried yet: the shrew. That would be cool: I can already picture it now. A girl is playing the role of Katherina in Shakespeare's
The Taming of the Shrew turns into the animal itself.
EDIT: Yes, the Tiktaalik and the shrew sound best in my ears...