Japanese demon

Yōkai (妖怪, ghost, phantom, strange apparition) are a class of supernatural monsters, spirits and demons in Japanese folklore. The word yōkai is made up of the kanji for "bewitching; attractive; calamity"; and "spectre; apparition; mystery; suspicious". They can also be called ayakashi (あやかし), mononoke (物の怪), or mamono (魔物). 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. Yōkai usually have spiritual or supernatural power, with shapeshifting being one of the most common. Yōkai that have the ability to shapeshift are called bakemono (化物) / obake (お化け).

Japanese folklorists and historians use yōkai as "supernatural or unaccountable phenomena to their informants". In the Edo period, many artists, such as Toriyama Sekien, created yōkai inspired by folklore or their own ideas, and in the present, several yōkai created by them (e.g. Kameosa and Amikiri, see below) are wrongly considered as being of legendary origin.

What is thought of as supernatural depends on the time period, but generally it is considered that the older the time period, the more phenomena were considered supernatural.

According to the ancient ideas of animism, a spirit-like being called a mononoke among other things was thought to reside in all things. It was believed that these spirits possessed various kinds of emotions, and if the spirit was peaceful, it was a nigi-mitama that brought about good fortune such as a good harvest, and if the spirit was violent, it was an ara-mitama that brought about ill fortune such as natural disasters and illness, and the ritual for converting ara-mitama into nigi-mitama was called the chinkon ("the calming of the spirits"). One's ancestors, other well-respected people, and sometimes even nature and animals depending on the area were considered nigi-mitama that became protective gods that received worship. At the same time, people also tried performing chinkon rituals in order to quell the misfortune-making beings from making any more misfortune and fear, the misfortunes and fears that were still unexplained in those eras. In other words, yokai-like beings can be said to be from the ara-mitama that weren't deified, failed to be deified, or stopped being deified.

...
Wikipedia