According to hyakumonogatari.com
“Yokai uses two kanji;
- 妖 (yo) which means “mysterious, bewitching, unearthly, weird.” It doesn’t really have a scary nuance to it, but more of the attraction to something beyond the normal. It can be used in words like yoka (妖花) meaning an ethereally beautiful flower, or ayashii (妖しい) meaning bewitching or charming.
- 怪(kai) which means “mystery, wonder, strange.” Kai has more of a sense of horror, or the bizarre. It is the same kanji used in kaidan (怪談) meaning “weird tales” and kaiki (怪奇) meaning “bizarre, strange, outrageous.”
Yokai means more than just monsters it literally means mysterious phenomena so it could range to anything from the paranormal to magical creatures. Shigeru Mizuki, the author of GeGeGe no Kitarō, breaks down the word Yokai in four categories: Kaiju which means mysterious beast or monster; Choshizen means supernatural; Henge means strange and tranform or shape-shifters; Yurei which means dim spirit or ghosts.
Some examples of what a yokai is includes kami (ghosts and gods), tanuki raccoon dog), kappa (river child), kitsune (fox), oni (demon or ogre), bakemono, ghosts, tengu are goblins, hebi, bakeneko (monster cat, ghost cat), inugami (dog god, dog spirit), and that is just to name a few.
One of the popular anime in Japan right now is Yo-Kai Watch which was .
If you would like to learn more about them here is an example of one story about the Aizuwakamatsu no Yurei
Japan's stories of many Yokai are very interesting so I highly reccommend reading them either from your local library or online.
Everyone have safe Halloween.