妖怪（ようかい）
Yōkai
The general word for the strange beings and happenings of Japanese folklore.
Yōkai joins two characters: 妖, bewitching or uncanny, and 怪, strange or suspect. In court chronicles of the eighth century the word named odd happenings at the palace, not creatures. For most of the Edo period people said bakemono for a monster; yōkai became the standard word only in the Meiji era, when the philosopher Inoue Enryō lectured on yōkaigaku and the folklorist Yanagita Kunio collected yokai names from the countryside, and it became the everyday word through the manga of Mizuki Shigeru in the 1960s.
The word is wide. It covers a creature with a body and a name, like the kappa; an animal that changes shape, like the fox; a tool that has come to life; and a sound on a night road that nobody can explain. What it usually excludes is the yūrei, the ghost of a particular dead person, though the line is not sharp.
Related yokai
Kappa
河童（かっぱ）
A child-sized water yokai whose strength lives in the water held by the dish on its head.
Kitsune
狐（きつね）
A 1930 account from Toyooka describes blurred blue or red foxfire moving restlessly from beyond the mountain to somewhere close by.
Azuki-arai
小豆洗い（あずきあらい）
A bean-washing sound repeats beside a dark stream, but the person making it cannot be found.
Karakasa-kozō
唐傘小僧（からかさこぞう）
An 1857 Osaka print shows a kabuki actor playing a one-legged umbrella monster — the clearest history of a yokai known through pictures rather than encounters.