Kappa(河童, "river-child"), alternatively called Kawatarō (川太郎, "river-boy") or Kawako (川子, "river-child"), are legendary creatures found in Japanese folklore. In Shintō they are considered to be one of many suijin. A hair-covered variation of a Kappa is called a Hyōsube (ひょうすべ).
Kappa are similar to Finnish Näkki, Scandinavian/Germanic Näck/Neck, Czech Vodník and Scottish Kelpie in that all have been used to scare children of dangers lurking in waters.
Kappas are typically depicted as roughly humanoid in form, and about the size of a child. Their scaly, reptilian skin ranges in color from green to yellow or blue. Despite their reptilian features, Kappa are said to have apelike faces and scraggly green hair.
Kappas supposedly inhabit the ponds and rivers of Japan and have various features to aid them in this environment, such as webbed hands and feet. They are sometimes said to smell like fish, and they can certainly swim like them. The expression kappa-no-kawa-nagare ("a kappa drowning in a river") conveys the idea that even experts make mistakes.
Their most notable feature is an indentation on the top of their head that holds water; this is regarded as the source of their power. This cavity must be full whenever a kappa is away from the water; if it spills, the kappa will be unable to move. Although they are reported to inhabit all of Japan, they are often said to be particular to Saga Prefecture.
Japanese folklore states that the kappa can be very polite, and can be disabled before it attacks by bowing. The kappa would bow back, and spill the water, disabling itself. Kappa are usually seen as mischievous troublemakers. Their pranks range from the relatively innocent, such as loudly passing gas or looking up women's kimonos, to the malevolent, such as drowning people and animals or kidnapping children.
One possible explanation for these creatures is the giant Japanese salamander which is rumored to grow up to eight feet in length and lives only in clean, clear streams (which is unfortunately where people like to play). Another is that it could be a new or prehistoric kind of amphibian, such as a temnospondyl.