I think the Ixions are creatures created by the Goddess much like how Cait Sith is an 'avatar' but technically is a small fragment of Altana's power, not a real Avatar per se. Atomos is another entity as it exist across space/time and is probably one of the Primordial entities of the Universe like the Celestial avatars that existed before the Mother crystal split. This is probably so since Altana can't impose her will directly on Atomos itself he is of equal or greater power.