Darknut

Originally appearing in the original Legend of Zelda game, the armored Darknuts are part of Ganon's elite fighting corps. They were some of the toughest enemies around, being equipped with shields that repelled all frontal attacks, doing a decent bit of damage, having plenty of stamina, and having an unpredictable movement pattern. They appeared in most dungeons, and nearly always in numbers, increasing their threat even further.

The Wind Waker
These armored knights reappeared in The Wind Waker, where their strong armor and massive blades proved a formidable match for Link. The Darknuts were also extremely resourceful and would engage in close quarters combat without a weapon or take advantage of fallen enemy's weapons if it had lost its own. Weak when attacked from behind, where their armor was strapped, Darknuts in this game had relatively few other weaknesses. Link soon discovered that the best way to deal with these powerful enemies was the Parry Attack, which would allow him to easily remove the Darknut's armor and avoid its sword. Finding a Darknut normally means the presence of the enemy scenario.

Multiple Darknuts
Encountered in later dungeons and in the Savage Labyrinth on Outset Island. While it may seem that multiple Darknuts pose a greater threat than one, the battle is essentially the same. Pick them off one by one with Link's parrying attack, and keep your guard up. It is assumed, however, that by the time you encounter more than one Darknut you will have a decent amount of hearts and the higher number should not pose a real threat.

Twilight Princess
Darknuts in this game are much harder than in Wind Waker. They start out with armor, a large sword, and a shield. When you land a hit on them, some of their armor comes off. Eventually their shield will fall too. Once you strip off all of their armor, they will throw their large sword at you. At this stage they use a smaller sword and wear only their under-armor chain mail making them much more agile. One Darknut is the mini-boss in the temple of time, while others commonly guard items such as keys or new parts of an area. In some cases you fight multiple Darknuts at a time which can be overwhelming.

Darknuts seemed to have been ranked by Ganon, and different ranks wore different amour and carried different weapons. Regular Darknuts wore grayish armor, sported a Viking-like helmet, and carried only a sword. Red Darknuts, otherwise known as Mighty Darknut, showed their higher rank via crimson armor, a crimson cape, a helmet that covered the entire face, and a crimson and yellow shield with their serrated swords.

In The Minish Cap, Darknuts returned again, this time with a Darknut who was even stronger than the Mighty Darknuts. This one was known as the Black Knight, a Darknut resembling a regular Darknut, only instead with a shield and cape. Everything about it was black as midnight, and it challenged Link as the mini-boss of the final dungeon in the game, Dark Hyrule Castle.

Quote from The Legend of Zelda: The Minish Cap Dark Nut figurine:

"Appears in Dark Hyrule Castle. These armor-clad soldiers are tough. Use your shield and rolling skills to find an opening in their defenses!"

How to kill
Wind Waker: Easy once you get used to it, simply get close enough for them to strike then use the quick (A) command to knock their armor off. Once you have done this you can do the same to there helmet (Under is a dingo's head) or just slash until they explode.

Twilight Princess: Slightly trickier. Throw a bomb at them just before it's about to explode, so it goes off behind them. This should remove some of their armor. Repeat this a few times until they throw their weapon at you and unsheathe a smaller sword. Now the even harder part begins. You need to get them to block an attack by swinging your sword at them. Once they block, go for an L-targeted jump attack (L-target then press A). When they block this, they should be stunned enough for you to administer some pain. Repeat until dead.

(See also: Mighty Darknut)