For me demons are allot like humans but very beautiful and with some animal characteristics, kinda immortal, and have the ability to change. Example. Dog demon may have a tail, fangs, ears *Or just pointed ears* as well she shifting form of a giant wolf thing or something.
I personally wanted to be a bat demon. My character would have fangs, pointed ears and wings that can go in and out of her back at her own will. No shifting form though. Basically she can pass for human.