Iron Knuckle

The Iron Knuckle is one of the largest, stockiest foes in The Legend of Zelda franchise, however, only appears in The Adventure of Link, Ocarina of Time, and Majora's Mask.

Characteristics
The Iron Knuckle is an enormous armored knight, equipped with a blunt or sword-like weapon capable of dealing immense damage. Though quite similar to Darknuts, Iron Knuckles are more powerful, less agile, and less present across the Zelda series. These foes are dueling elites, incorporating much of the same swordplay technique Link does during battle, to great effect; furthermore, these creatures are known for their unmatched defensive capabilities and long vitality. In their debut, The Adventure of Link, these knights take to caverns and dungeons, and appear in three varieties of increasing strength: orange, red, and blue, with the Blue Iron Knuckles being capable of shooting Sword Beams. Likewise, in later titles, Ocarina of Time and Majora's Mask, Iron Knuckles also come in three, increasingly powerful colors: silver, white, and black.

The Iron Knuckle gained mainstream popularity as a miniboss in Ocarina of Time and Majora's Mask, in the former appearing as a facet of the plot at the Spirit Temple. These foes swing huge axes, capable of taking three hearts from Link at every swing, not to mention their grand size and armor coverage. If Link is hit by this ax, he will lose quite a large amount of life energy.

Rebonakku
In The Adventure of Link, an Iron Knuckle called  is the boss of the Island Palace. He rides a ghostly horse. Link must use the Down Thrust initially to damage him. After enough damage is done, he dismounts his horse, and can then be fought as a normal Iron Knuckle. The trick is for Link to attack while standing and ducking down so that the boss can't block every attack with his shield. The mounted knight is also fought twice as a miniboss in Three Eye Rock Palace (he guards the Cross the first time).

Plot Significance in Ocarina of Time and Majora's Mask
In Ocarina of Time, Nabooru is captured and brainwashed by Koume and Kotake, a pair of Gerudo witch sisters. Koume and Kotake then force the brainwashed Nabooru to don armor and protect the entrance to the final chamber in the Spirit Temple as an Iron Knuckle. When Nabooru is defeated in her Iron Knuckle form, the armor falls away and reveals her, apparently releasing her of her hypnosis. The fact that the other two Iron Knuckles in the Spirit Temple dissolve completely when they are defeated suggests that they were animated by magic and not hypnotized people, like Nabooru.

In Majora's Mask, there are only three Iron Knuckles: two are beneath the Ikana Graveyard on the first and second day respectively: one is guarding the resting place of Flat and the other is guarding a Piece of Heart. The third is on the Moon in the hide and seek game played by Link while in his Hylian form.

Weaknesses
Because these foes are as tall as Link and are experts at defensive strategy, using a shield they constantly move up or down to block Link's attacks in The Adventure of Link, Link must jump and crouch stab at its head to inflict damage. In Ocarina of Time and Majora's Mask, the armor of the Iron Knuckle can be removed, providing two stages to every battle. While wearing armor, they're slow and impervious all attacks save for sword swings and bombs. Once enough armor is removed, they become much quicker, but are now weak to arrows, opening up the opportunity for a successful long-range strategy.

Captain N: The Game Master
The Rebonakku appears in two episodes of Captain N: The Game Master. He is first shown in "Quest for the Potion of Power" after Link, Princess Zelda, and Kevin arrive to the chamber of the Potion of Power in Island Palace, where the Rebonakku reveals himself to be the guardian of the potion. Link and Kevin argue as to how they should go about defeating the Iron Knuckle, and before they even manage to inflict damage upon the armored foe, Princess Zelda is taken by the Rebonakku and Link realizes that he must fight alongside Kevin to be able to defeat it. Together, Link and Kevin fire at the Iron Knuckle, causing its horse to disappear and giving Zelda a chance to free herself. The three heroes then shoot at the Iron Knuckle one more time to finally defeat it.

In "Having a Ball", another Rebonakku confronts them while Link, Princess Zelda, Kevin, and Princess Lana are exiting the tunnel that leads to the Island Palace. Kevin tries to shoot it with his Zapper but it has no effect, so they jump out of the way and allow the Iron Knuckle to fall into a nearby lake along with its horse.

The Legend of Zelda comic
The Rebonakku from The Adventure of Link appears in the Missing in Action story of the Legend of Zelda comic, although he is only known as an Iron Knuckle. Like in the game, he is the guardian of the recorder in the Island Palace. When Link and Zelda arrive to the palace in search of the Recorder, they are confronted by the Iron Knuckle. After a grueling battle, Link defeats his foe, knocking its helmet off. It turns out that it is a human, and not a monster under the armor. The man explains that an old king charged him with protecting the  treasures within the palace, and that he was to stop anyone who was unworthy. He tells the duo that by defeating him, they are free to take what they want, and he shows them a secret wall that contains the magic flute.

Trivia

 * In Ocarina of Time, the armor within which Nabooru is encased looks slightly different than that of the other Iron Knuckles fought in the game.
 * There is Gold Iron Knuckle armor on display in the Curiosity Shop in Majora's Mask.
 * Onox, the main antagonist of Oracle of Seasons, highly resembles the appearance of an Iron Knuckle. The only real difference is that Onox wields a flail while Iron Knuckles tend to use axes.
 * Iron Knuckles from Ocarina of Time and Majora's Mask wear a cloth tunic bearing Gerudo patterns, similar to those seen on Ganondorf's and Twinrova's clothing.
 * If Link positions himself behind an Iron Knuckle and the camera is panned through its armor, the face of a Gerudo is visible through its helmet. This is most likely only used as a model for the creature but it can be theorized that Ganondorf turns his people into Iron Knuckles. This also works in Majora's Mask, and then more of the body is visible.
 * In the N64 game  the sound clip used for Geodude's cry is very similar to the grunts of an Iron Knuckle from Ocarina of Time.