In Robert A. Heinlein's science fiction novel The Rolling Stones, flat cats are a species of Martian animal.
Description and origin
In the chapter "Free Enterprise", the character Mr. Angelo, a shopkeeper on Mars, introduces Castor and Pollux Stone to a flat cat:
Angelo tickled it with a forefinger; it began to purr like a high-pitched buzzer. It had no discernible features, being merely a pie-shaped mass of sleek red fur a little darker than Castor's own hair. "They're affectionate little things and many of the sand rats keep them for pets - a man has to have someone to talk to when he's out prospecting and a flat cat is better than a wife because it can't talk back. It just purrs and snuggles up to you."
The boys take the flat cat onto the family space ship, where it soon has eight "kittens", each of which soon gives birth again, until the ship is overwhelmed with flat cats. The family solves the problem by rounding up the flat cats and putting them into the storage hold at low temperature, where they hibernate. They are later revived and sold to miners in the asteroid belt.
Heinlein's flat cats are often said to have been the inspiration for the tribbles of the Star Trek episode "The Trouble with Tribbles". (Heinlein himself said he may have gotten the idea from Ellis Parker Butler's 1905 story "Pigs Is Pigs".) David Gerrold, the author of the episode, claims that he had read the Heinlein book years before writing his screenplay and was not consciously aware of the similarities until Desilu/Paramount conducted a routine studio clearances review following an inquiry by Kellam de Forest, its primary in-house researcher. This prompted a contact with Heinlein who admitted the similarities but also graciously waived all rights.