From eWeek:

[Apple’s 2003 OS] Panther will mark the third significant upgrade to Mac OS X since its debut—and the fourth big cat from Apple. The initial Mac OS X release bore the internal code name Cheetah, and Mac OS X 10.1, which shipped in September 2001, was referred to internally as Puma although neither moniker was ever publicized. (Mac OS X 10.2 Server was code-named Tigger, sources said, another sobriquet that never saw the light of day.)