Kappa, alternatively called Kawatarō or Kawako, are legendary creatures, a type of water sprite 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 .
Kappa are similar to Finnish Näkki, Scandinavian/Germanic Näck/Neck, Slavian Vodník and Scottish Kelpie in that all have been used to scare children of dangers lurking in waters.
It has been suggested that the kappa legends are based on the Japanese giant salamander or "hanzaki", an aggressive salamander which grabs its prey with its powerful jaws.
Kappa 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. Kappa 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.
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, kidnapping children, and raping women.
As water monsters, kappa have been frequently blamed for drownings, and are often said to try to lure people to the water and pull them in with their great skill at wrestling. They are sometimes said to take their victims for the purpose of eating their livers or their shirikodama (尻子玉?), a mythical ball inside the anus. Even today, signs warning about kappa appear by bodies of water in some Japanese towns and villages. Kappa are also said to victimize animals, especially horses; the motif of the kappa trying to drown horses is found all over Japan. In these stories, if a kappa is caught in the act, it can be made to apologize, sometimes in writing.
Kappa are also known as ravishers of women. An 18th-century ukiyo-e image by Utamaro depicts kappa raping an ama diver underwater. In his Tōno Monogatari, Kunio Yanagita records a number of beliefs from the Tōno area about women being accosted and even impregnated by kappa. Their offspring were said to be repulsive to behold, and were generally buried.
It was believed that if confronted with a kappa there was but one means of escape: Kappa, for one reason or another, obsess over being polite, so if you were to gesture a deep bow to a kappa it would more than likely return it. In doing so, the water kept in the lilypad-like bowl on their head would spill out and the kappa would be rendered unable to leave the bowed position until the bowl was refilled with water from the river in which it lived. If a human were to refill it, it was believed the kappa would serve them for all eternity.
Kappa are not entirely antagonistic to humankind, however. They are curious of human civilization, and they can understand and speak Japanese. They thus sometimes challenge those they encounter to various tests of skill, such as shogi or sumo wrestling. They may even befriend human beings in exchange for gifts and offerings, especially cucumbers, the only food kappa are known to enjoy more than human children. Japanese parents sometimes write the names of their children (or themselves) on cucumbers and toss them into waters believed to be infested with kappa in order to mollify the creatures and allow the family to bathe. There is even a kind of cucumber-filled sushi roll named for the kappa, the kappamaki.
Once befriended, kappa have been known to perform any number of tasks for human beings, such as helping farmers irrigate their land. They are also highly knowledgeable of medicine, and legend states that they taught the art of bone setting to humankind. Due to these benevolent aspects, some shrines are dedicated to the worship of particularly helpful kappa. Kappa may also be tricked into helping people. Their deep sense of decorum will not allow them to break an oath, for example; so if a human being can dupe a kappa into promising to help him, the kappa has no choice but to follow through.