List of (almost) all way to change form!

[Metastachydium]

I saw your post after the Yuan-ti addition. I agree it is barely for PC but I still added for completion sake.

I don't mind that at all! What I'd like to quickly point out instead is that the CS mountebank PrC gets the at-will Alter Egos and the DComp mountebank base class's Infernal Guise is the one with uses/day.

With that out of the way, here's a few more PrCs that might be of interest to you:
Fist of the Forest^CC gets Feral Trance (Su) 2/day in 3 levels; it's a kind of quasi-rage (which stacks with rage) that gives the fist claws and fangs to claw and bite with;
similarly, Bloodclaw Master gets Shifting (Su) 2/day in levels; it's pretty much what the Shifting of shifters does, but with a single option to pick (a tigerlike thing with claws); and
Scorpion Heritor^Sand has Scorpion Form (Su), gained at level 3, which is essentially 1/day Polymorph into a tiny monstrous scorpion only; and Tail of the Scorpion (Su), gained at level 9, which is kind of self-explanatory. It likewise works 1/day.