Oni is a term used in Japanese folklore to describe a type of demon, ogre, or troll. They are believed to live in caves or deep in the mountains and are known for their strength and fear-inspiring appearance. Oni are often depicted as having red or blue bodies with horns and fangs, and are armed with metal clubs called kanabō. They are generally considered to be foreign in origin, perhaps introduced into Japan from China along with Buddhism.

The name "oni" comes from the Japanese reading of the Chinese character for ghost, and it was initially thought to refer to any spirit, monster, or ghost. However, it gradually evolved to mean the oni we know today. In Japanese mythology, oni is an ancestral demon, and it was believed that they could transform themselves into familiar animals or disguise themselves as people.

Although oni have been described as frightening creatures, they have become tamer in modern culture as people tell less frightening stories about them. In recent years, there have been stories about oni that depict them as creatures who only want a bit of company, playing on their similarity in shape to humans and giving them human characteristics such as loneliness and companionship.

In summary, oni is a term used in Japanese folklore to describe a type of demon, ogre, or troll that is known for its strength and fear-inspiring appearance. They are believed to live in caves or deep in the mountains and are often depicted as having red or blue bodies with horns and fangs, and are armed with metal clubs called kanabō.