Oni and Youkai
Along with magic, psychic powers, ki, and other supernatural forces that exist in Tsuwamono-era Japan, the land is also populated by mysterious creatures which lurk in darkness and escape human understanding. The umbrella term for these monsters is Youkai, although there are many different types. Some are hostile, some are peaceful and friendly. Some Youkai are close to human in their behavior, while others are alien and confounding. The only common trait between them is that all Youkai, regardless of appearances and actions, are of a supernatural existence and have a sort of sentience above that of animals.
A particularly common type of Youkai are the Oni, muscular humanoid brutes with horns and tough, leathery skin. The Eastern Oni, those in the lands East of the Emperor’s palace, are known to be primitive, destructive, and gigantic, towering over a man and smashing everything in their path with their giant metal clubs. Western Oni, in Chugoku and beyond, are, in contrast, said to be more civilized, appearing closer to humans in size and build, and utilizing sophisticated clothing and weapons. Despite this, both types are warriors by nature, and quick to anger.