In Japanese folklore kappa are one of the most well-known and easily recognisable yokai. They are a type of water goblin, resembling a small reptilian creatures about the size of a small child. They have green skin and a distinctive bowl-like dip on the top of their heads. Kappa live in lakes and rivers waiting to attack unsuspecting passers-by and can be very dangerous.

If you encounter a kappa it will either tear out your insides in search of a mystic jewel, drag you to a watery death, or both. Be sure to have some cucumbers ready if you happen to meet one as it's their favourite treat and can often distract them long enough to allow you to escape.

Alternatively, a deep bow should do the trick. Out of politeness, kappas will bow in return and the water will run out of the dip on the top of their heads, causing them to lose their energy. Folklore around the world has many tales of monsters lurking in waterways waiting to attack those who come close enough, and if you are in Japan, it's the kappa you need to watch out for. 
'A Hundred Tales from Japan & China: Shirafuji Genta Watches Kappa Wrestle' - Tsukioka Yoshitoshi, 1865.

Throughout time these tales of water monsters have been an effective way of keeping children away from water to avoid drowning. Despite the gruesome folklore, kappas are very popular in modern Japan and are often portrayed as cute little creatures. Today you can find kappa soft toys and even costumes. 

Kappa statues below are from the Kappabashi homewares district in Tokyo.

More kappa art below.

Utagawa Kuniyushi
'The fisherman Urashima Tarot looking down at a kappa' - Utagawa Kunisada, 1846.
'Humorous Scenes at the Famous Places of Tokyo - Taken by Kappa Monster' - Utagawa Hiroshige, 1883.

'Kappa' - Issunshi Hanasato, 1843.

'Yazama Shunroku Mitsukaze Fighting a Kappa' - Utagawa Kuniyoshi,1857.

'Kappa' - Kan Kosaki

'Hokusai Manga Vol. 12 - Smoker and kappa Monster' - Katsushika Hokusai, 1814-1819
Keyamura Rokusuke struggling with three kappa on the bank of a river in which a fourth kappa also is swimming - Utagawa Kuniyoshi.