Autochthones and Gegenoi

In Greek myth, the giants and other races were spawned by Gaia. There are even a few different names for this: autochthones, gegenees, spartoi. 13th Age borrows this same concept to explain where its depiction of orcs come from: they spring fully-formed and holding weapons from gashes in the earth, then go forth to destroy civilization as Gaia demands. This concept is really useful because it sidesteps having to worry about things like orc non-combatants and so forth. So I would like to apply that concept to most monsters, at least the humanoids, beastmen and so forth that parties are typically expected to slaughter as a matter of course.