The Oni (鬼) are legendary Japanese monsters that are heavily involved in the nation's folklore - regarded as demons, ogres or trolls they are very popular characters both in traditional stories and in modern work, though they can be friendly they tend to be antagonistic.
Oni are normally depicted as hideous, gigantic humanoids with sharp claws, unkempt hair and two long horns growing from their heads - they can have varied numbers of eyes or extra fingers and toes and have skin of any color imaginable, thouh red and blue seem to be the most common.
Oni tend to wear tiger-skin loincloths and carry iron clubs, which is where the expression "oni with an iron club" came from (which means to be invincible).
Indeed Oni were seen as virtually unbeatable, the only way to really disperse them was with loud noise and celebrations - even to this day some villages in Japan perform these rituals to drive away Oni spirits.
As well as the ogres of today Oni have been believed by some to be gods, ghosts or other supernatural beings that caused unpleasant things to occur and originally they were said to be invisible to human eyes or formless - in modern times this ethereal being has been largely forgotten and replaced with the creatures described above.