Azazel

A cat who made a deal with the Devil.
In exchange for its Earthly soul, it can summon the powers of hell.
When it does, long horn-like spikes poke through it's body.
The longer they are, the more power the cat has summoned.
Why would anyone do this?
This cat had grown up with a lot stress.
It was left all alone as a kitten and grew up by himself
living in trash bins and trying to avoid predators.
One day, after a particularly close call with a pack of mutts,
this cat had had enough. It was sick of being scared for its life
all the time.
The Devil, of course, sensed the great anger and distress in this
small, lonely cat, and offered to make the deal.
Now, this cat is a fearless creature of the night, in fact,
it's almost a tyrant.
With it's new found strength and frightful appearance,
they torment everything and everyone who had tormented it.
Needless to say, there aren't any stray dogs in its part of town anymore.