Yokai are one of those uniquely Japanese concepts that don’t easily translate into English or American culture.
An imperfect definition for the uninitiated: yokai are supernatural beings, most of whom are up to no good, some who are neutral, and a few who bestow good fortune to the mortals who happen to cross paths with them.
The term is a large umbrella for a variety of creatures including demons, ghosts, and spirits. Some were originally mortal but were turned into yokai because of their actions, thoughts, or emotions. Interestingly, the same yokai may be described quite differently depending on which region of Japan you pose the question.
That’s a result of yokai springing from folklore, which is locally-specific.
Although yokai stories have been around for well over a thousand years, their popularity exploded in the Edo period (1600’s).
Obviously, there’s a Wikipedia page about them (is there anything that site doesn’t have a page for?), as well as an incredibly awesome site dedicated to indexing dozens of them [side note: I love this site and its original artwork depicting yokai…so much so that I bought two paperback books from the author. Both are never outside of arm’s reach when I’m writing.]
But now there’s also a museum dedicated to yokai!
The Miyoshi Mononoke Museum just opened (perhaps not so coincidentally) in the city of Miyoshi, which happens to be the setting for a popular yokai story.
If I make it back to Japan someday, this is definitely on my must-see list!