Term used by the Wilmarth Foundation, the anti-Old Ones foundation in Brian Lumley's Titus Crow novels, to refer to all the other little nasties of Cthulhu's particular species of the Old Ones. It is believed that there as many as five hundred of 'em down in the Pacific, living in anything up to half a million square miles' worth of city.
Cthulhu Himself is referred to as the 'race-father', which implies that one of his children must have, at some point, copulated with. Err. Something. No, I'm not going any further down that route, and for those sick individuals who wish I would... go find some tentacle porn or something.
Although it's possible that Lumley is using the race-father term as only a symbolic one, as he does later have his informer - Professor Wingate Peaslee - speculate that Cthulhu may just have been the first of his species to arrive from the stars. Regardless, Cthulhu is their progenitor, and they all live something around three hundred fathoms down.
Information from Brian Lumley, 'The Transition of Titus Crow'.