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.
In the Otogizōshi, familiar tales such as Urashima Tarō and Issun-bōshi also appeared.
The next major change in yokai came after the period of warring states, in the Edo period. As a result, kashi-hon shops that handled such books spread and became widely used, making the general public's impression of each yokai fixed, spreading throughout all of Japan.
In this period, toys and games like karuta, sugoroku, pogs frequently used yokai as characters.
Thus, with the development of a publishing culture, yokai depictions that were treasured in temples and shrines were able to become something more familiar to people, and it is thought that this is the reason that even though yokai were originally things to be feared, they have then became characters that people feel close to.
Yōkai that have the ability to shapeshift are called bakemono (化物) / obake (お化け).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.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.