Genbu: a black tortoise representing winter and the north

This is Genbu, one of four gods which came to Japan from China. They are scarcely remembered at all today except in various fantasy mangas...

Genbu is a black tortoise representing winter and the north. Genbu is said to have done battle with a serpent, and is frequently shown either entwined with the snake or - as here - with the snake on his back.

http://antiquesinjapan.com/Buddhism3.html