Kitty Comets can be extremely rare in earlier versions of the mod, but with the introduction of Comets they are slightly more frequent (at least when they are by themselves). There is a one percent chance (default setting) that hundreds of Kitty Comets can come down at once. Be sure to have a Meteor Shield setup to prevent them from damaging your environment when that happens.
Kitty Comets leave behind a Comet Kitty, which are adorable little things with space helmets on their heads. They are stronger than ocelots, keep away alien creepers, and are immune to fire. They can also breed with ocelots and their own kind to birth more Comet Kitties.