Angler Fish
Link encounters this boss in Angler's Tunnel, the fourth dungeon in Link's Awakening. Angler Fish is the Nightmare guarding the Surf Harp.
Link fights it under water. It is easiest to defeat it by attacking the glowing spot on its head with your Sword, but the Bow and Bombs will work as well. As soon as you land into the water swim to it's head and swing the sword as fast as you can and it will die without even showing what it has to offer.