ROOTS RACES -- According to magicians, before human beings appeared there were root races on Earth. The first Root Race were invisbale being made of fire-mist; the Second were gaseous creatures; the Third were brainless, egg-laying monsters on whom the Diluvials are based; the Fourth were Atlanteans, including the Rmoahals.
If you follow the next link. It will take you to a page... that after a short explanation outlines in a way satisfactory to me... these four races above. Though there some discrepancies with other information I have read. In some versions the Lemurians ( The Third!) who in one account are described as very much like jelly fish and on other accounts three eyed giants with bi-lateral feet. Also described as having the most incommon with natives of Gondwanaland. It's hard to separate fact from fantacy.
In accordance with Slaine these are closely related to the exoskeletoned Dilluvials -known to melt rock using their horns- the Cythrons plucked from the streams of time to take down the Everliving Ones at Dinas Emrys.
Okay, I think apart from all that. It sort of fits and the people incharge of Runequest Slaine - And it's Supplements - might find abit more inspiration here.
As I beleive Slaine owes some of his mysterious background to a connection to the four Root Races.