Oni (鬼) is the broad term used for ogres, trolls, and the like. They are frequently portrayed as large, grotesque humanoid monsters who wield a club and have sharp fangs or claws.
Ko-oni is a small oni.
Oni Hitokuchi is a yōkai who lives in storehouses, originally from the Tales of Ise. Women who have been placed in storehouses to hide will disappear by dawn. He eats them in one bite.
Rajōmon no Oni are oni who dwell in the Gate of Rashōmon.
Tearai Oni is a giant who straddles the mountains of Shikoku on either side of the sea in order to wash his hands.
Shuten Dōji is the leader and strongest of all oni. He was said to have once been a handsome man who spent all his time studying and ignored the love letters women sent to him, resulting in many of them committing suicide of broken hearts. As a result, he tried to read the letters but was engulfed in smoke and turned him into an oni.
Ibaraki Dōji is either a man who was cursed into becoming an oni by licking blood from a love letter, or the adopted son of a barber who eventually became an oni after tasting the blood of his customers. After running away from home, he met Shuten Douji and became his servant.
Kidōmaru is Shuten Douji's son who is half-oni and half-human. He tried to avenge his father's death by destroying the man who had killed him. Concealing himself inside of a cow's corpse, he sought to ambush his father's killer but was outwitted and ultimately defeated.