An oni is a Yōkai from Japanese folklore. The word oni is often translated to "demon" but oni also have a lot in common with trolls or ogres from western folklore and mythology. They are known for their propensity for violence, murder and cannibalism.
Physically they are usually large humanoids. They have very specific faces with very important traditional importance. They are usually depicted with red, blue, black, green, yellow or white skin and tend to wear loincloths often made of tiger pelts.
Depictions can vary wildly but they are usually portrayed as hideous hulking creatures. Sometimes they have one or multiple horns and sometimes multiple eyes.
They usually carry around large spiked clubs called kanabō clubs.
The name "oni" comes from the on'yomi reading of the character (隠) which means "to hide" or "to conceal". This is due to oni having the tendency of hiding behind things.
Tradition and Culture
In February there is a festival known as Setsubun. During Setsubun you throw roasted soybeans indoors or out of your home and shout "Oni wa soto! Fuku wa uchi!", ("鬼は外！福は内！", "Oni go out! Blessings come in!"). This is derived from the medieval ritual of tsuina which is a year-end rite to drive away oni and other spirits.
There is also a charm made of holly leaves and dried sardine heads in the Tottori prefecture used to protect against oni.
The oni have long been very popular in Japanese tradition and feature in many well known fairytales such as Momotarō (Peach Boy), Issun-bōshi, and Kobutori Jizan.