The history of OS X started with a bear, not a cat: in September of 2000, Apple presented Kodiak. For 30$, users could purchase this first public Beta of Mac OS 10.
The official OS 10.0, Cheetah, followed 6 months later. It featured Apple's brand new user interface Aqua and the first versions of apps like TextEdit, Preview, Mail, and QuickTime.
The new OS X demanded an (at that time: whopping!) 128 MB of RAM and 800 MB of disk space.