Arrghus

Arrghus (ワート, Wart) is the guardian of the second Crystal in the Water Gate 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 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 will launch into the air and attempt 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 will attempt to crush Link from above again, but persistence leads to its defeat. After its destruction it will leave behind a Heart Container and Link will be 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 (also known as Onoffs) 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 will sometimes unleash 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 will attack 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 will disintegrate and leave Link with the Yellow Maiden.

Trivia

 * Wart is a very similar boss 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 Arrghus in A Link to the Past. In Japan, Arrghus' name is Wart,  meaning that both Arrghus and Wart are simply different names for the  same monster.