They are demons. Most can pass themselves off as human beings, but are identifiable by their slitted red eyes (which they can’t normalize. They are forced to wear colored contacts, which quickly dissolves because of the liquid venom present in their eyes, tongue, and sweat). Their weapons of choice are the iron kanabo (a club) or a giant sword. In their original forms they have wild hair that is usually white, two to four long horns growing from their foreheads, and skin color varies between each Oni.
Oni are born when truly wicked humans die and end up in one of the many Buddhist Hells, transformed into Oni. Occasionally, when a human is so utterly wicked that his soul is beyond any redemption, he transforms into an Oni during life, and remains on Earth to terrorize the living. These transformed Oni are the ones who pose the most danger to humankind.
There are Okina Oni (Greater Oni) and Shimo Oni (Lesser Oni).