(covers information from several alternate timelines)
The feline appearance of Caitians included long manes and a tail. While the skin of some Caitians was covered with fur that varied in color from orange to black, other Caitians were without fur or trimmed down completely. Female Caitians grew long Human-like hair on their heads. (Star Trek: The Animated Series; Star Trek IV: The Voyage Home; Star Trek Into Darkness) The voices females speak with have a soft, cat-like purring quality. Some Caitians did not find it necessary to use footwear. (Star Trek: The Animated Series)
The name of the species comes from a biography of Lt. M'Ress published by Lincoln Enterprises in 1974, which claims that the Caitians are related to the Kzinti. This is corroborated by extensive commentary by Larry Niven, author of most Known Space books, primarily found in his short story books "N-Space," "Playgrounds of the Mind", and "The Patchwork Man."
The Lincoln Enterprises biography of M'Ress expanded on the Caitians, stating that they tended to be small and sinewy, with their females delicately dainty. Their planet, Cait, was described to have atmosphere, mass, and gravity all near Earth normal. There was only one other planet in the system, and it was uninhabited. 
M'Ress is also referred to as a Caitian in the non-canon audio story "Passage to Moauv".
Set in 2409, the computer game Star Trek Online features the Caitians as a playable species of the Federation faction, which also employs ships of unique Caitian design, e.g., the Atrox Carrier and the Stalker Stealth Fighter. The game includes several Caitian NPCs, including Enterprise-F Tactical Officer and Security Chief Lieutenant Kyona.