Mōryō or mizuha (魍魎, 罔両, or 美豆波), is a collective term for the spirits of mountains and rivers, trees and rocks, as well as mononoke that live in places like cemeteries, or kappa and various other yōkai.
The term mizu no kami also exists to refer to them.
Originally, they are nature spirits in China. In the Huainanzi, it is said that the "mōryō have the shape of a child from one to three years old, are dark red in color, have red eyes, long ears, and beautiful hair."
In the Bencao gangmu, we find the statement that the "mōryō like to eat the entrails of the dead. They then proceed to perform the "Rites of Zhou," take a dagger, and enter the grave hole where they bring destruction.
In their true nature, the mōryō are afraid of tigers and oak trees and are called 弗述. They go underground and eat the brains of the dead but it is said that when an oak tree is pressed against their neck, they die. These are the ones called mōryō".
With regard to their habit of eating the entrails of the dead, in Japan the mōryō are sometimes considered to be the same as the yōkai who steal the corpses of the dead, i.e., the kasha and examples can be seen where kasha-like stories are classified as "mōryō."
In Negishi Shizumori's Edo period essay Mimibukuro, a government official named Shibata has a loyal servant who one night declares, I am not a human being, but a mōryō and resigns.
When Shibata asks the servant the reason, the servant replies that it is because since it is now his turn to play the role of stealing a corpse, he must go to a certain village.
The next day, the servant has disappeared and at a funeral in the village he mentioned, some dark clouds suddenly gather and when the clouds have disappeared, it is said that the corpse has disappeared from the coffin.