Oni are defiant spirits of chaos and mischief from Japanese folklore. The word Oni can be translated to mean ogre or troll, but unlike the trolls we see in traditional western belief, these trolls are very intelligent and have magical powers. Some believe their magic is so powerful that their limbs are able to instantly re-attach themselves to the Oni’s body once removed, making them very formidable opponents. Not all Oni are evil either. Since they love a good battle, some may take it upon themselves to banish evil spirits just for the fun of it.
Some sources say that Oni like the taste of human flesh, and from this belief came the belief that Oni are spirits of death. In Buddhism, the Oni guard the gates between various hell and death realms (watch out Dante, there are 18 levels). Here’s a link with some pictures of the different stages of hell, and you can see Oni torturing the people here. Some sources say that Oni predate Buddhism in the far East however and were later incorporated into Buddhism to assist with assimilation. Their role changed from guardians to evil beings with the rise of the samurai class in Japan. Stories then featured samurai facing off against the evil Oni (knight vs dragon).
While there are supposedly female Oni (and they are supposedly growing rarer and rarer), Oni were originally pure spirits who did not keep up population by sexual reproduction. As legends have it, when men die while they are overwhelmed by excessive anger, they transform into Oni.
Oni have horns, tusks, and fangs. They have either red, blue or black skin and are often clad in a tiger skin of sorts. In more recent times, Oni statues are used to keep out bad spirits, kind of like gargoyles in the West.
There is a legend about a man named Shoki who would battle the Oni, and you can read more about that here