Yoshiyuki Iikura, Associate Professor at Kokugakuin University, writes:Folklore of demons and monstersAccording to

The roots of demons are said to date back to the Kojiki (712) and the Nihon Shoki (720).

Is Yamata no Orochi the root of demons?

He suggested that Susanoo's slaying of Yamata no Orochi was the origin of many subsequent demon-slaying legends (in other words, the origin of demons). Upon hearing this, some may have thought, "Isn't Yamata no Orochi a giant snake?", which doesn't seem to be associated with demons. I am one of those people. However, as I continued reading, I realized that Yamata no Orochi was born in the Hi no Kawa River (Hiikawa) and is said to have been a symbol of natural disasters that frequently cause flooding, and the hero Susanoo who defeats it, is defeated by the evil god. This composition is certainly a depiction of demon slaying, and since no earlier writings can be found, I thought it might not be wrong to say that this is the root of demons. However, personally, I don't think that "demons" are something so grand, but rather "humans" who have fallen into darkness, so I felt it was a bit of a stretch to say that Yamata no Orochi is the root of demons just because of the composition. So, what are the roots of demons from the perspective of a yokai lover? This is something I'd like to look into in the future.

A magnificent view. It's like the flowing dragon of Yao.

Since the introduction of Buddhism in the Asuka period?

It is well known that those coming from the direction of Ushitora in the Heian period were demons (rebellious people), but what about the times before that? During the Asuka period, when Japan had many exchanges with Korean countries such as Baekje, the allied forces of Japan and Baekje were defeated by the allied forces of Tang and Silla, but the people of Baekje were washed ashore in what is now Sado in Japan, and the Japanese people were scared because they could not communicate with each other. The theory that the "Oni" (demon) was born from this connection with Buddhism, which was introduced at that time, is what I found most convincing. As a yokai lover, I would like to strongly promote the idea that demons are human-sized. I think the image of demons is based on the Ushitora (ox horns, tiger pants) of the Heian period and the mask of the Hosouji in the Oinashiki ceremony.

Popping out?

I would like to continue to study the roots of demons in depth and try to find my own solutions.