In traditional Japanese folklore a kappa (河童, "river-child")—also known as kawatarō (川太郎, "river-boy"), komahiki (駒引, "horse-puller"), with a boss called kawatora (川虎, "river-tiger") or suiko (水虎, "water-tiger")—is a reptiloid kami with similarities to yōkai.

Accounts typically depict them as green, human-like beings with webbed hands and feet and turtle-like carapaces on their backs.

A depression on the head, called a "dish" (Japanese: sara), retains water, and if this is damaged or its liquid is lost (either through spilling or drying up), a kappa becomes severely weakened.

[1] They are often accused of assaulting humans in water and removing a mythical organ called the shirikodama from their victim's anus.

[2] The name kappa is a contraction of the words kawa (river) and wappa, a variant form of 童 warawa (also warabe) "child".

[6] Kappa are said to be roughly humanoid in form and about the size of a child, inhabiting the ponds and rivers of Japan.

[7] They are typically greenish in color[8] (or yellow-blue[9]), and either scaly[10][11] or slimy skinned, with webbed hands and feet, and a turtle-like carapace on their back.

[21] Lafcadio Hearn wrote of a story in Kawachimura near Matsue where a horse-stealing kappa was captured and made to write a sworn statement vowing never to harm people again.

[5][22] In many versions the kappa is dragged by the horse to the stable where it is most vulnerable, and it is there it is forced to submit a writ of promise not to misbehave.

One notable example of this method is the folktale of a farmer who promises his daughter's hand in marriage to a kappa in return for the creature irrigating his land.

Since the dish of water on their head is a Kappa's greatest weakness, should it spill, a human who refills it will also earn lifelong friendship.

Once befriended, kappa may perform any number of tasks for human beings, such as helping farmers irrigate their land.

[32] There were also festivals meant to placate the kappa in order to obtain a good harvest, some of which still take place today.

These festivals generally took place during the two equinoxes of the year, when the kappa are said to travel from the rivers to the mountains and vice versa.

The nearby Jōkenji [ja] In Tōno, there is a Buddhist temple that has komainu dog statues with depressions on their heads reminiscent of the water-retaining dish on the kappa's heads, said to be dedicated to the kappa which according to legend helped extinguish a fire at the temple.

A green human-like being named a vodník is widely known in western Slavic folklore and tales, especially in the Czech Republic or Slovakia.

[4] In Japan, the character Sagojō (Sha Wujing) is conventionally depicted as a kappa: he being a comrade of the magic monkey Sun Wukong in the Chinese story Journey to the West.

In their explicitly commercial conceptions, yōkai are no longer frightening or mysterious—the DC Card Kappa, for example, is not a slimy water creature threatening to kill unsuspecting children but a cute and (almost) cuddly cartoon character.