Kappa are probably one the most famous mythical creatures in Japanese folklore. And no wonder, since Japan is surrounded by water! For those that do not know what a kappa is, it is a creature similar to some sort of river child. They are often depicted with a somewhat human form, green skin, a turtle shell on its back, a beak for its mouth and what is called a sara on its head. Sara is a dish on its head that has to be full of water if the kappa wishes to go on land. It is said to be the source of its powers, and if the sara were to become empty, the kappa could die.
Kappa are known to be curious and mischievous (just like the tengu), but can also be quite malicious. There are tales of kappa sometimes kidnapping children and raping women. On the other hand though, they are also known to be extremely polite and enjoy challenges. So if you ever need to challenge a kappa, make sure to bow, so that he returns your bow and his sara empties itself. Another way to ward of a kappa: feed it a cucumber. Because for some reason the kappa loves cucumber!
However, just as they can be malicious, if a kappa befriends humans they can actually be quite useful by helping farmers with irrigation and so on.
Some believe to have spotted kappa in real life, but who knows…