Also known as Uka no Kami (宇賀神) a harvest and fertility deity from Japanese mythology, with a serpent's body and the head of either a bearded man or a woman and related to Benten -often it's depicted with her or near her or even as one entity aka Uga Benten (宇賀弁天) resulting in one of those mixtures of Shintoism, Buddhism and mythology that abound in the Japanese religious landscape. Here in the entrance of Bentendo Temple in Ueno.
(For a bigger version of this picture both in color and black and white, check my "Japan Arekore" set on Flickr).