In traditional Japanese folkloristics, yokai are classified (not unlike the nymphs of Greek mythology) by location or phenomenon associated with their manifestation.Yokai are indexed in the book Sogo Nihon Minzoku Goi (綜合日本民俗語彙, "A Complete Dictionary of Japanese Folklore") The ancient times were a period abundant in literature and folktales mentioning and explaining yokai.Yasaburo was originally a bandit whose vengeful spirit (onryo) turned into a poisonous snake upon death and plagued the water in a paddy, but eventually became deified as the "wisdom god of the well (井の明神)." Kappa and inugami are sometimes treated as gods in one area and youkai in other areas.From these examples, it can be seen that among Japanese gods, there are some beings that can go from god to youkai and vice versa.Literature such as the Kojiki, the Nihon Shoki, and various Fudoki expositioned on legends from the ancient past, and mentions of oni, orochi, among other kinds of mysterious phenomena can already be seen in them.In the Heian period, collections of stories about youkai and other supernatural phenomena were published in multiple volumes, starting with publications such as the Nihon Ryōiki and the Konjaku Monogatarishū, and in these publications, mentions of phenomena such as Hyakki Yagyō can be seen.Clicking one will show its picture, description and location.
Violent spirits, ara-mitama, brought ill fortune — including illness and natural disasters.One's ancestors and particularly-respected departed elders could be deemed nigi-mitama, accruing status as protective gods and receiving worship.Animals, objects and natural features or phenomena were also venerated as nigi-mitama or propitiated as ara-mitama — depending on the area.Yōkai range diversely from the malevolent to the mischievous, or occasionally bring good fortune to those who encounter them.
Often they possess animal features (such as the kappa, which is similar to a turtle, or the tengu which has wings), other times they can appear mostly human, some look like inanimate objects and others have no discernible shape.
Meanwhile, depictions of yokai in emaki and paintings began to standardize, turning into caricatures and softening their fearsome natures.