Here's the big cat names Apple have used (just internally at first, but as official codenames from Mac OS X 10.2 onwards), according to wikipedia:
10.0: Cheetah 10.1: Puma 10.2: Jaguar 10.3: Panther 10.4: Tiger 10.5: Leopard
The problem is this: there are precious few big cat names left. There's "Lion" and, er, that's it. After that, you've got Ocelot, Bobcat, and you're done.
This is the same mistake as companies who decided to name servers after planets, which is fine as long as you only have a few - Mercury, Venus, Mars etc. - but gets problematic very quickly.
