Kappa ("river-child") alternatively called Kawataro or Kawako are legendary creatures, a type of water sprite found in Japanese folklore. 

Kappa are usually seen as mischievous trouble makers. Their pranks range from the relatively innocent, such as loudly passing gas, or looking up women's kimonos, to the malevolent, such as drowning people and animals, kidnapping children, and raping women. 

As water mosters, lappa have been frequently blamed for drownings, and are often said to try to lure people to the water and pull them in with their great skill at wrestling. They are sometimes said to take their victims for the purpose of eating their livers or their shirikodama, a mythical ball inside the anus.

Even today, signs warning about kappa appear by bodies of water in some Japanese towns and villages.

-"Kappa (folklore)" Wikipedia.

Oh Japanese. You can make porn about anything. Forever inspiring.