A kappa is in Japanese folklore, a river Demon with the body of a tortoise, the limbs of a frog, and the head of a monkey. His head has a hollow at the top containing a strength-giving fluid. He lives in the water and comes out in the evening to eat. He sucks the blood of horses and cows through their anuses. He also drags humans into the water and sucks out their blood through their anuses. Humans can outwit him by being civil to him and bowing.
This act forces Kappa to return the bow; his life fluid spills from his head and he loses his strength. He can also be placated by throwing cucumbers with the names and ages of family members into the water where he lives. If a human is challenged to single combat with a kappa, it is essential to accept and hope that it will not be able to keep its head erect during the encounter.
The human can then extract a promise from the kappa while it is in its weakened state.
SOURCE:
Encyclopedia of World Mythology and Legend, Third Edition – Written by Anthony S. Mercatante & James R. Dow– Copyright © 2009 by Anthony S. Mercatante