Unholy Necromancy, batman!
Well, the Horned Reaper can only be summoned via some form of magic, but he does leave a body unlike the imps. I'm going to say that they are not a magical creation, but a being not of this world. Perhaps they make their home in the realm of the dark gods, and serve them? It would seem to make sense.
I like the concept of Skybird Trill's Reaper being Horny.