"Old Ones" was a name used during the Doctor's education for the most powerful species of the Dark Times. He had read of their "rise", which was to be contrasted with the generally peaceful demeanour of most species of their era.
In their heyday they were "giants of the universe", but, by the time the Doctor crossed paths with them, all of them had been reduced to mere "remnants". Among those late-surviving Old Ones were the Exxilons, the Eternals, the Jagaroth and the Racnoss. (PROSE: The Knight, The Fool and The Dead)