I don't really think there would be a problem naming OX S 10.9 "Cougar." In fact, Apple should. It's a fairly big, strong cat, and the name connotes power and ferocity. Much more so than "Lynx."
Next, Apple has had the trademark on Cougar (and Lynx) since 2003. No problem there.
Finally, there is that minor business with the informal use of the term Cougar that applies to certain kinds of older women.
I don't think that's really an issue for Apple. First, Apple easily survived Jay Leno's short-lived joke about the iPad mini. Second, these kinds of jokes don't have much staying power. The first time someone tries it, it's geek-cute. The second time a writer tries it, it's awkward and puerile.
By the time the third try comes around, the writer just looks creepy and stupid. The whole thing is dropped, and Apple can get on with the prestige of a big cat name. And reclaim the higher ground.
I really don't see a problem.
(A list of current and previous OS X cat names is in wikipedia.)
Cougar image via Shutterstock.