(*OS X (*10.9*) COUGAR!!!*)
Honestly, I think they always intended to use Cougar at some point along the way and that word was co-opted so they didn't, leaving them one short. They may jump ahead to whatever the naming scheme will be for OS 11.
They have used all the (non-extinct) big cat names besides Cougar. That leaves two plausible possibilities, OS X Sabertooth and OS X Leo.
There aren't any left. Hence why I posted earlier that Apple's likely to use something else.Well aren't there any other cat names left? I don't think we'll see OS XI for another 7-8 years.