Oni are creatures from Japanese folklore, variously translated as demons, devils, ogres or trolls. They are popular characters in Japanese art, literature and theatre. Depictions of oni vary widely but usually portray them as hideous, gigantic, creatures with sharp claws, wild hair, and two long horns growing from their heads. They are humanoid for the most part, but occasionally, they are shown with unnatural features such as odd numbers of eyes or extra fingers and toes. Their skin may be any number of colors, but red and blue are particularly common. They are often depicted wearing tiger-skin loincloths and carrying iron clubs, called ''kanab''.This image leads to the expression ''oni with an iron club'' that is, to be invincible or undefeatable. It can also be used in the sense of ''strong beyond strong'', or having one's natural quality enhanced or supplemented by the use of some tool.
Oni were originally invisible spirits or gods that caused disasters, disease, and other unpleasant things. These nebulous beings could also take on a variety of forms to deceive (and often devour) humans. Thus, a Chinese character meaning ''ghost'' came to be used for these formless creatures.
The invisible oni eventually took on its modern, ogre-like form, from creatures imported by Buddhism, such as the Indian rakshasa and yaksha, the hungry ghosts called ''gaki'', and the devilish underlings of Enma- who punish sinners in Jigoku (Hell).
Some villages hold yearly ceremonies to drive away oni, particularly at the beginning of spring. During the Setsubun festival, people throw soybeans outside their homes and shout ''Oni wa soto! Fuku wa uchi!'' (Demons out! Luck in!). Monkey statues are also thought to guard against oni, since the Japanese word for monkey, saru, is a homophone for the word for "leaving". In Japanese versions of the game tag, the player who is ''it'' is instead called the ''oni''.
In more recent times, oni have lost some of their original wickedness and sometimes take on a more protective function. Men in oni costumes often lead Japanese parades to ward off any bad luck, for example. Japanese buildings sometimes include oni-faced roof tiles called onigawara which are thought to ward away bad luck, much as gargoyles in Western tradition.
Tattoos of Oni or Oni masks are popular with admirers of Japanese traditions and designs. Over time, the oni tattoo has come to represent good luck and to ward off evil spirits.