The kappa is a water dweller resembling a scaly monkey with webbed hands.
A Kappa will grab and strangle waders in its pond (PA8). According to Snape, the Kappa is commonly found in Mongolia (PA9), but the Care of Magical Creatures textbook states that the Kappa is a Japanese creature (FB).
A Kappa lives in shallow ponds and rivers and feeds on human blood, but can be overcome either by tricking it into bowing (resulting in all the water stored in the top of its head draining out, depriving it of strength), or by throwing it a cucumber with a certain person's name carved into it, which persuades it not to harm that person (FB).
Kappas are water sprites from Japanese folklore. They are said to have bowl-like depressions on the tops of their heads and they keep these filled with water. This is the source of their strength. To overcome a kappa, according to folklore, one must bow to it, enticing it to bow in return and spill the water from its head. (The Lexicon page 172).