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, then they are vulnerable to Link's sword. When all of the Arrgi are gone, Arrghus starts crawling over the ground very fast. In this state he can be easily defeated using Link's sword. After his destruction, Link gets a Heart Container and the Second Crystal in which one of the seven maidens is imprisoned.

Wart
Wart is a similar mini-boss on Majoras Mask.