I think maybe some kind of demon. Obviously one that has some kind of power over men (hence Diamond's creepy skill to fog over Miles' brain) they need to feed on human blood. I do not think they need to become some grotesque creature but rather maybe just over exaggerated facial features? The fangs like Karah thinks she saw? Something along the lines of that I think would be good. That way we aren't really following the times with vampires and lycans. I don't know about The Strain because they haven't told that much of the story yet. But I have heard them being referred to as vampires. So we cant be told we are following along with that.