Looks like I was right! Mountain Lion it is!!!
Anyone care to wager on what 10.9 will be? Cougar?
I really doubt they will follow Mountain Lion with Cougar - another name for the exact same species. Yes, there was Puma, also another name for this species, but that wasn't marketed. Puma was nothing more than an internal name, and was never meant to be widely known. Following Mountain Lion with Cougar would just confuse a lot of people, and wouldn't be that great of marketing.
Also, and this is just my opinion, I think they purposely went with Mountain Lion over Cougar to avoid any, uh,
unwanted connotations "cougar" carries.
But really, the only real "big cats" they haven't used are Lynx and Bobcat, both of which barely meet the "big" in "big cats." They've got to move on to OS XI as some point, and drop the cat names. I'm rooting for birds next. OS 11.0 Hawk, 11.1 Falcon, 11.3 Bald Eagle, etc.