Kappa are legendary creatures; a type of water sprite found in Japanese folklore. However they are also considered to be a part of Cryptozoology, due to claims of sightings. It could be the Japanese giant salamander.
In Irish folklore, the Dobhar-chu is a water creature that resembles an otter or a dog, and may be part fish. It is also a cryptid, and there are alleged sighting of the Dobar-chu as recently as Possible Puck sightings, while in the Northern Kingdom?
The Kelpie of Irish and Scottish lore. The kelpie will come to land as a beautiful horse, luring humans to it. Once a person gets on it's back, the kelpie races to the water, transforming to it's true water demon form, and drowns its victim.