Cheetah - fastest animal on land...can run 70+ mph!
But I would hope it wouldn't be thought of as "cheater."
That's been used as well.
Mac OS X 10.0 (Cheetah)
Mac OS X 10.1 (Puma)
Mac OS X 10.2 (Jaguar)
Mac OS X 10.3 (Panther)
Mac OS X 10.4 (Tiger)
Mac OS X 10.5 (Leopard)
I just read on Wikipedia (not the most reliable source), that Apple has registered "Lynx" and "Cougar" as trademarks, so there are the two big possibilities. I vote for Cougar.
Liger