Recently my brother and I visited an antique store in New Braunfels and ran across a few strange little ceramic figures, of the sort that were very popular in the '50's. They were in the shape of froggy little creatures with turtle shells and a monkish fringe of hair. I was able to identify them as kappas, mythological Japanese creatures that inhabit ponds, streams, and other bodies of water.
Kappas are about the size of a human child (kappa means "river child"; it's alternate names are kawataro "river boy" and kawako "river child"). Their bodies are described as monkey- or frog-like, with a turtle's carapace. Their faces have been described as apelike, but often have a turtle's beak or duck's bill. They have scaly skin, webbed hands and feet, and are green or blue in color. What they all have is that fringe of hair, and in the middle of that fringe a bowl or depression filled with water.
The kappa has been described as a sprite or water imp; in the Shinto religion it is considered one of the suijin, or water gods, not unlike the nymphs of Greco-Roman mythology. In Japanese popular use they are Nursery Bogies, like the English Jenny Greenteeth or Peg Powler, warning children away from bodies of water in which they would be in danger of drowning. Kappas are said to be fond of eating little children, but also occasionally adults; the way to placate them is to throw cucumbers with the names of family members etched into them into the water. Cucumbers are the only things kappas like eating more than humans.
The other way to overcome a kappa is to bow deeply before him. A kappa is so traditionally polite he will bow in return; this will spill the water from the top of his head, and without this water he is powerless. In this state he will remain until you fill the bowl again, and then he will be your friend and might even give you help. In this manner, according to old tales, the Japanese first learned the art of bone-setting from the kappa. Besides being learned in medicine, the kappa is said to be fond of engaging humans in contests such as sumo wrestling and shogi ("Japanese chess").
In the old days kappas were more threatening figures, but their image has mellowed in modern times, and they pop up all over the place in popular culture. The turtle-like Koopas in the Mario games are said to be based on kappas; there is a cucumber-stuffed sushi called kappamaki; they have appeared in numerous anime series. As a matter of course Hellboy encounters a kappa among the other traditional Japanes creatures in Hellboy: Sword of Storms. Knowing what a kappa is has helped me understand what was otherwise an inexplicable image in a lot of anime shows.