Arrghus

Arrghus is the guardian of the second Crystal in the Swamp Palace in A Link to the Past and the Eastern Temple in Four Swords Adventures. Arrghus is also a mini-boss, known by its Japanese name Wart, in the Great Bay Temple and the Secret Shrine in Majora's Mask.

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 Arrgi 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, in A Link to the Past 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, with a look more similar to its Majora's Mask counterpart. To defeat this stone version of the 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.

Wart
Arrghus, known by its Japanese name Wart, is a huge eye mini-boss that rolls around trying to hit its enemies in the Great Bay Temple and the Secret Shrine in Majora's Mask. It attacks and is defeated like its previous incarnations. At the start of the battle, Wart is hovering slightly above the ground, completely surrounded by a mass of bubbles, rendering it immune to any damage. It is a time-consuming enemy. The sheer amount of bubbles on it make it a long struggle to pierce its shield, as it is constantly moving, along with Wart, which will differentiate from retreating to trying to press itself against Link in an attempt to cause damage. The bow is not a good idea at this point, as there are simply too many bubbles. Any offensive weapon is capable of removing the bubbles, although the best is the bomb, which knocks many bubbles off at once. Using the Hookshot is another way of removing the bubbles, although it is a time-consuming way. Other way is use the deku nuts, because they can destroy many bubbles in one shot and damage Wart. The bubbles are incapable of being destroyed while on Wart, only knocked off, which makes them even more of a pain. When an opening is finally made, and Wart's eye is no longer blocked by bubbles, the bow is the way to go. However, any projectile will work. After enough shots, Wart will enter stage two, forcing its surviving bubbles off its body and begin rolling around wildly. First finishing off any bubbles, then attack Wart with the bow. After a few more shots, Wart will be finished.

A Link to the Past comic
A creature resembling Arrghus is featured in Chapter 9 of the A Link to the Past comic. Fought in the Swamp Palace just like in the game, the monster is shown as a bright red jellyfish that attacks with tentacles, but no eyeballs are shown. In the comic, as Link makes his way to the imprisoned Maiden, a large, tentacled monster attacks him. He thrusts himself with the Master Sword into the monster and defeats the creature, which spits out Link from inside and gives him a  chance to destroy the floating crystal where the maiden is.

Trivia

 * It is possible that the name "Arrghus" is a reference to Argus Panoptes, an all-seeing hundred-eyed giant from Greek mythology.
 * Wart has a resemblance to Vitreous, who is killed in the same manner.
 * A frog-like character named Wart also appeared in Doki Doki Panic (or, Super Mario Bros. 2). This character differed drastically from the Wart of Majora's Mask. Interestingly, this Wart made a cameo appearance in Link's Awakening, with his Japanese name Mamu Mamu.
 * The Deku Toad from Twilight Princess bears some similarity in the way it acts to Wart. Like Wart, it has many eye-like objects on its body (though these are actually its eggs), but not nearly as many as Wart did. It also starts the battle on the ceiling, like Wart. Interestingly enough, as mentioned above, the final boss of Super Mario Bros. 2 is a giant toad named Wart.