How about a hierarchy of demons?
Where lesser demons are more aggressive and less intelligent, or to flip it around, the higher the demon is in the hierarchy the more intelligent and scheming they are.
Maybe the lesser demons look more and more like animals and and the higher demons take on the form of a human, now I am not saying that they will lock like humans, just their general shape.
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Or scratch that, all of it besides the first line. The demons resides in a mirror image world right so then what about mirror image version of ourselves, something like they have taken the form of what it it we fear the most. then making species as that as a guideline?