Oni are humanoid, ogre-like creatures. Descriptions of oni vary greatly, but these yōkai are generally depicted as having claws, messy looking hair, fang-like tusks, and two horns and they are traditionally male. They are occasionally portrayed as having an odd number of eyes or extra fingers and toes. Their skin can be any number of colors, but it is most commonly depicted as either red or blue. They are often shown wearing tiger-skin loincloths and carrying iron clubs.

Oni are created when a wicked human dies and goes to Jigoku (Hell). In Jigoku, oni are responsible for punishing the humans who are sent there that weren't quite wicked enough to become oni themselves. The punishments they carry out include peeling off skin, crushing bones, and other horribly agonizing acts.

When a living human becomes so wicked that their soul is beyond redemption, they will transform into an oni while they are still alive and remain on Earth to terrorize the living. These are the oni that are most commonly featured in legends. They are said to cause natural disasters, spread diseases, and deceive people in order to eat them,

Some villages hold yearly ceremonies to drive away oni. During the Setsubun festival the day before the beginning of spring, people throw soybeans outside their homes and shout "Oni wa soto! Fuku wa uchi!" (meaning "Oni go out! Blessings come in!"). Monkey statues and holly are also thought to guard against oni.

In the present day, oni are usually seen as less wicked than they were in the past. In fact, they are sometimes presented as warding off bad luck. For this reason, men in oni costumes often lead parades in Japan in the hopes of warding off bad luck themselves. Japanese buildings sometimes include oni-faced roof tiles for the same reason.

FUN FACT: Oni are prominently featured in children's games in Japan. For example, in Japanese versions of tag, the player who is "it" is instead called the oni. Japanese children also enjoy playing kakure oni (hidden oni), also known as hide-and-seek.

Examples in various media:

King Yemma (Dragon Ball and Dragon Ball Z)

Tarakudo (Jackie Chan Adventures)

Name unknown, nicknamed Oni's head (Inuyasha)

Suika Ibuki (Touhou Project series)