Caitians are natives of the planet Cait, a Class M world and member of the United Federation of Planets. Caitians are felinoid bipeds, with a strong resemblence to the Terran lion. They have leonine facial features, paws, manes (on both genders), and tails, but their body structure is otherwise clearly humanoid.
Caitains are rarely seen in Star Trek, because no amount of makeup would make human actors look like proper Caitians. The species was created for the Animated Series, for the character Lieutenant M'Ress. One of the members of the Federation Council in Star Trek IV: The Voyage Home looked as if he could have been a Caitian, but that's unconfirmed.
Little is known about Caitians, as M'Ress has appeared in no official sources, and only in a few episodes of the Animated Series. Conjecturally, the species is related to the Kzinti but are much less militaristic; Caitians are peaceful but can be formidible warriors when needed. It's generally agreed that Caitians share the playful and curious spirit, the hunter's instinct, and the stealth and agility of Terran felines.