I think the Exo race might be old man made war machines designed to fight what ever tried to exterminate the human race, or maybe they're repurposed worker robots with advanced AI.
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I was originally thinking advanced A.I., but the more I thought about it the less it made since to me. In a universe that brings both Sci-Fi and Fantasy together, that seemed to "just Sci-Fi" for something as important as a character race. Regardless, the possibility that the Exo are just robots is still a plausible theory.