By the way, why are they called "YOKAI" ?

The word "YOKAI" first appeared in the beginning of the 1st century A.D. It meant "mysterious and abnormal phenomenon."

In Japan, "YOKAI" first appeared in the Nara era. In "Zoku-Nihon-ki", there is a description that "A purifying ceremony was held as YOKAI often appeared in a palace."The word "YOKAI" gained its popularity in the Meiji era, by the researchers who got interested in monsters and ghosts that were familiar to the people.