The Lost Race was a sentient race of Earthlings whose evolution predated the Three Races and has since become extinct, although Greg Weisman has yet to reveal when or how.  In fact, he writes, "it's hard to give a category to something that currently I have no intention of discussing. But "Lost Race" seems as good a moniker as any -- as a place-holder." 
What is known is that the Lost Race, gargoyles, humans, and finally Oberon's Children appeared on Earth in that order. The Lost Race is in fact from Earth, and Greg denies their having any contact with extraterrestrials.  Although they eventually became extinct, they were still around when Oberon's Children first evolved, as they were aware of their existence.  Brooklyn encountered this race on his time-travels to the past during his TimeDancer adventures. Greg confirmed that the Lost Race has left behind relics and artifacts from their civilization.  It is speculated by fans that these may include the ruins in the Archmage's Cave, notably the Megalith Dance, although Greg refuses to confirm or deny the possibility.