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. It will try to use a ramming attack, fling enemy fish at you, and drop debris on you, all easy to dodge if you just sit there and hack.

Link encounters this bloated monster in Jabu-Jabu's belly. This is actually fought on dry land. Link has to shoot it with the Seed Shooter, then strike it in the antenna. After a certain amount of time, the fish will repuff itself and bounce around, shooting bubbles. After several rounds, it will explode.