Obake / Bakemono
Literally, "transforming thing." This refers to any type of unnatural being. Comprises yokai and yurei, and can also be used more generally to refer to anything that is weird or grotesque. Foxes, and
tanuki are bakemono.
Yokai
Literally, "bewitching apparition." This encompasses a wide spectrum of ghouls, goblins and monsters, some of which are frightening, some amusing and even cute, and many which are bizarre. Kappa are yokai. Yokai usually appear at dawn or dusk.
Yurei
Literally, "dim/hazy/faint spirit." Spirits of the dead who remain among the living for a specific purpose, usually to seek vengeance. Yurei generally appear between 2 and 3 AM.
Oni
"Demons" or "ogres." Ferocious creatures with horns and fangs. Often found manning the gates of the various Buddhist hell realms and performing some of the tortures that take place in them.