Gangi kozō

Gangi kozō (Japanese: 岸涯小僧 or がんぎこぞう, meaning "riverbank priest boy") are monkey-like water spirits which inhabit rivers in Japan.

According to Mizuki Shigeru, the name gangi kozō can be written with another set of kanji, 雁木小僧. These characters can mean “stepped pier” or “gear tooth” depending on the context. This writing reflects both the habitat of the gangi kozō as well as its mouth full of sharp teeth, which resembles a toothed gear.

Myths & Legends
Gangi kozō live along the riverbanks, where they hunt fish. They normally stay away from people and rarely interact with them, but occasionally encounter fishermen along the rivers they inhabit. When meeting a gangi kozō, fishermen often leave their largest, cheapest fish on the riverside as an offering.

According to many legends, kappa transform from river spirits into hairy mountain spirits when the seasons change, which may be what these creatures are. The specific details differ quite a bit from place to place. However, in Yamaguchi prefecture, there is a hairy mountain spirit called a takiwaro which transforms into a water spirit called an enko (a variety of kappa).

Appearance
Their bodies are covered in hair, and the hair on their head resembles the the bobbed  okappa  hair style once popular among children in Japan. Their most notable features are their webbed hands and toes, and their long teeth which are sharp and jagged like files. They are close relatives of the much more well-known kappa.

Origin
Gangi kozō are not encountered outside of the riverbanks, and there may be a good reason for this; according to one theory, they are a transitional form of kappa. Some folklorists believe that the gangi kozō is a kind of takiwaro, and thus is merely a transitional form of a kappa. This would explain why so little is known of them.

Gangi kozō do not appear in any local legends, though stories of very similar-looking yokai do. The first and only written record of them is in Toriyama Sekien's yokai encyclopedias. It is therefore possible that gangi kozō was made up by Toriyama Sekien based on the numerous legends of transforming kappa.