Demons

Origins
True demons are beings made of pure energy originating in a small pocket dimension. This dimension, if physical, would be no larger than a basketball and more dense than a black hole, and would theoretically collapse the physical universe. Described as being "an unending existence as nothing but thought, unimaginable heat, and unwavering darkness" this dimension contains the energy equivalent of twenty trillion suns.

Physical
Beings of pure energy, demons are not limited in the physical forms they can take. Their natural form is typically humanoid, with some animalistic traits - horns, claws, hooves, tusks, tails, wings, or tentacles are all quite common. With time and practice, a demon can learn to take the form of nearly anything. Many find it useful to master a smaller, less intimidating form, like a faerie or a human child.

Social
Demons are generally social beings, although they avoid contact with their own kind whenever possible. Strong demons are capable of stealing the energy of weaker demons, or even absorbing them completely. As demons are ageless and immortal, they seek out long-lived creatures as companions, typically befriending serpentine dragons or vampires.

Pseudo-demons
Most commonly mistaken for a true demon is the incubus/succubus, due to their physical appearance.

The wolf demon is not a true demon either, but the spirit of a werewolf that has mutated into another form of energy no longer requiring a living host.