By "yokai phenomenon" I mean the way anything, anyone or even concepts/ideas can become magical, animate, sentient and possibly predatory, as commonly depicted in Japanese mythology and modern fiction. For example traditional Tsukumogami are lanterns, umbrellas, swords, walls, sandals, anything that has been used/treasured for a long time, or abandoned/abused in some unjust way. It's heavily implied and in some manga outright stated that yokai are the result of human mental projection, by projecting human traits upon the world the world in turn becomes personified by yokai.
This obviously is wildly unrealistic and totally supernatural so I'm not looking for any justification for the existence of this phenomenon, it's totally unjustifiable, irreducible and unreproducible. There's no manufacturing yokai, there's no predicting yokai except in the broadest sense (i.e. you can't stop the phenomenon by predicting somewhere at sometime it will occur) there's no hard rules about what a yokai can or cannot be.
Yokai can be beneficial or malevolent, they can be annoying, murderous, gift giving, life saving, or totally unconcerned with people. They don't breed, they don't need to eat but they can, they don't seem to die of old age and if you kill one it'll either revert to what it was or the remains will disappear the moment you look away. Dissecting them yields nothing, there's flesh but its nonsensical, an untrained artist's interpretation of biology, how and why they do the things they do cannot be deduced by reason.
I'll provide more details if needed.