I suspect if he's going to get into fleshing out "oni" as a people at some point, it'll be when Yamato returns, and they won't really be direct transplants of oni from Japanese folklore, but just an unrelated people who happen to have horns and get
called that by others. Could be wrong of course, but he seems to be sticking to keeping these such figures symbolic within the series, except as legends. (I'd bet Kaidou's people probably got pushed into being used for war a lot like he did, but were treated as canon fodder and less able to resist than he was.)
My next-best guess is that Mars' mythical DF is going to be
Suzaku, btw. I have guesses for one or two others, but that's the one I feel has the most recognizable outline.