The Meridian housecat was a feline lifeform native to planet Meridian. They are very large animals which have a symbiotic relationship with smaller organisms living within their bodies. These symbiont lifeforms play a vital role in the housecats's reproductive processes.
Meridian housecats bear little resemblance to Terran cats, and the discovery of these animals led to the phasing-out of the term "housecat" for the Humans's domesticated pets. (TOS novel: Prime Directive)