This is the Shark, the second enemy in The Candy Box 2. On 14/10/13 aniwey posted on the blog: "In the meantime, here's a cute shark waiting for you to slay him!".
Some sharks have a coloured fin. Defeating these will gain you the magical red shark fin, then the magical green shark fin, and finally if you last long enough you will find a magical purple shark fin, which grants access to a 1 time use per quest black magic spell.