In Japanese, they are called “kawauso” . In Japanese folklore, they fool humans like the fox (kitsune) and tanuki. In the Noto region, Ishikawa Prefecture, there are stories where they shapeshift into beautiful women or children wearing checker-patterned clothing, and if a human attempts to speak to one, they will answer “oraya” and then answer “araya,” and if anybody asks them anything, then they say cryptic things like “kawai,” and there are also dreadful stories like the one in the Kaga Province (now Ishikawa Prefecture) where an otter that lives in the castle’s moat would shapeshift into a woman, invite males, and eat and kill them.
In the kaidan, essays, and legends of the Edo period like the “Urami Kanawa” , “Taihei Hyaku Monogatari” , and the “Shifu Goroku” , there are tales about strange occurrences like otters that shapeshift into beautiful women and kill men.
In the town of Numatachi, Asa District, Hiroshima Prefecture (now Hiroshima), they are called “tomo no kawauso” and “ato no kawauso”, and it is said that they would shapeshift into monks and appear before passers-by, and if the passer-by tries to get close and look up, its height would steadily increase until it became a large monk.
In the Tsugaru region, Aomori Prefecture, they are said to possess humans, and it is said that those possessed by otters would lose their stamina as if their soul has been extracted. They are also said to shapeshift into severed heads and get caught in fishing nets.
In the Kashima District and the Hakui District in Ishikawa Prefecture, they are seen as a yōkai under the name kabuso or kawaso, and they perform pranks like extinguishing the fire of the paper lanterns of people who walk on roads at night, shapeshift into a beautiful woman of 18–19 years of age and fool people, or fool people and make them try to engage in sumo against a rock or a tree stump It is said that they speak human words, and sometimes people would be called and stop while walking on roads.
In the Ishikawa and Kochi Prefectures, they are also said to be a type of kappa, and there are stories told about how they engage in sumo with otters. In places like the Hokuriku region, Kii, and Shikoku, the otters themselves area seen as a type of kappa. In the Kagakushū, a dictionary from the Muromachi period, an otter that grew old becomes a kappa.
In an Ainu folktale, in Urashibetsu (in Abashiri, Hokkaido), there are stories where monster otters would shapeshift into humans, go into homes where there were beautiful girls, and try to kill the girl and make her its wife.