There's been quite a few games using the Polymorph system. I think it technically started with Mazes, and was then followed by a number of family-friendly mini RPGs like The Excellents, Business Wizards, Horseshoe Academy, and Nancy Druid. Let's see, there's also Pigeon's Eleven, The Very Good Dogs of Chernobyl, Savage Sisters, Return to the Dark Tower (technically Mazes in the Return to Dark Tower boardgame setting), and of course the new version of Kobolds Ate My Baby, and the new releases of Scurvy Buggers and Sentai and Sensibility.
Most of the Polymorph games have been on the simpler side like Rebel Scum, the main differences being in how you define the dice roles, the action results (which actions are important to the setting, and what number results give you successes), and any special abilities or items you might have. They often play around with what the 1 and Max Roll results mean.
Mazes has been the most complex of them all, with a whole magic system, different abilities, advancement. In particular, though, has been its use of the Light/Dark die (the D12) which determines how risky things are.
Then Kobolds Ate My Baby has been the most divergent in rules, using ONLY D6s (though what you call them makes a big difference), and having innumerable amusing ways of dying.