Arrghus

Arrghus (ワート, Wart) is the guardian of the second Crystal in the Swamp Palace (second dungeon of the Dark World) in A Link to the Past and the Eastern Temple in Four Swords Adventures.

Characteristics
Arrghus is a large, one-eyed jellyfish which is surrounded by several miniature polyps called Arrgi. These Arrgi and the creature's ability to float make it invulnerable to attack. Its method of attack is to send the Arrgi swirling outwards. The Arrghi can be pulled away with the Hookshot, and then destroyed with the Master Sword. When all of the Arrgi are gone, Arrghus launches into the air and attempts to crash straight down upon Link, then it speeds across the shallow water of the room. In this state, it is unprotected and can be attacked with Link's sword or the Bow. After each hit, it attempts to crush Link from above again, but persistence leads to its defeat. After its destruction, it leaves behind a Heart Container, and Link is presented with the second Crystal in which one of the Seven Maidens is imprisoned.

Orange Arrghus
The orange Arrghus is the guardian of the Green Four Sword in the Palace of the Four Sword, an additional dungeon created for the Game Boy Advance version of A Link to the Past. Unlike the original Arrghus, this alternate version is surrounded by Bari in much the same way as Barinade in Ocarina of Time. As well as being more resiliant and able to deal more damage than the original Arrghus, the Bari sometimes unleashes their electricity when Link pulls them away from the monster, causing him damage as he attempts to destroy them. After the Bari have all been destroyed, Arrghus attacks in much the same way as before, but this time he travels faster and is able change direction and summon swarms of Kyameron, making him arguably the most difficult of the bosses that guard the elements of the Four Sword.

Stone Arrghus
The Stone Arrghus appears as the boss of Eastern Temple in Four Swords Adventures. To defeat the stone version of this Boss, Link must use his Boomerang to pull the rocks to him and then hack them with the sword. Once all the small rocks have been destroyed, the large, gigantic eye attacks Link. Once it has been hacked at enough, it disintegrates and leave Link with the Yellow Maiden.

Trivia

 * Wart is a mini-boss very similar to Arrghus that makes an appearance in Majora's Mask as the middle boss of Great Bay Temple. He attacks and is defeated in a very similar manner to Stone Arrghus in A Link to the Past. In Japan, Arrghus' name is Wart, implying that both Arrghus and Wart are simply different names for the same monster. Also similar to Arrghus is Vitreous, who also appears in A Link to the Past.
 * It is possible that the name "Arrghus" is a reference to Argus Panoptes, an all-seeing hundred eyed giant from greek mythology.