Translation: crazy bones
Diet: none; it is powered solely by vengeance
Appearance: A kyōkotsu is a ghostly, skeletal spirit which rises out of wells to scare people. It is wrapped in a ragged shroud, with only its bleached skull and tangled hair emerging from its tattered clothes.
Behavior: Kyōkotsu are formed from bones which were improperly disposed of by being discarded down a well. The bones may come from a murder or a suicide victim, or someone who died after accidentally falling into a well. The lack of a proper burial—and the egregious disrespect shown by discarding bones in this manner—creates a powerful grudge in those bones. This transforms the deceased into a shiryō. Like other ghosts, they pass their grudge on to those they come in contact with. A kyōkotsu lies at the bottom of its well until it is disturbed, then it rises up to curse anyone unfortunate enough to be using the well.
Origin: Kyōkotsu was invented by Toriyama Sekien for his book Konjaku hyakki shūi. In his description, he writes that this yōkai’s name is the origin of the word kyōkotsu, which means fury and violence. While there is a word in a local dialect of Kanagawa which does match this description, there is no evidence actually linking it to this yōkai. It is more likely that Toriyama Sekien—who was fond of wordplay—actually created this yōkai based on words in local dialects and just made up a false etymology to make the story more interesting.