Arrghus

Arrghus is the guardian of the Second Crystal in the second dungeon in the Dark World of A Link to the Past.

Arrghus is a large, one-eyed jellyfish which is surrounded by several miniature polyps named Arrgi. To defeat him, these must first be destroyed. They can be pulled away with the Hookshot, and then slashed with the Master Sword. When all of the Arrgi are gone, Arrghus starts crawling over the ground very fast, and also jumps up high in the air in an attempt to squash Link. In this state, he is unprotected and can be easily destroyed using Link's sword. After his destruction, Link receives a Heart Container and the Second Crystal in which one of the seven maidens is imprisoned.

Wart
Wart is a very similar miniboss that makes an appearace in Majora's Mask. He is the mini boss in the Great Bay Temple. Interestingly enough, he is defeated in the same exact manner, with the exception that arrows must be used on it's single eye.