A large and powerful enemy from The Legend of Zelda: Ocarina of Time. They are basically knights, with gigantic axes and very heavy armor. They attack slowly, but their reach is very long (about twice the length of Link's reach with the Biggoron's Sword), and is very damaging (you lose about 4 hearts per hit). In addition, your shield will not protect you from the attack, while their armor is heavy enough that it will take about 10 good hits with the Biggoron's Sword to defeat them.
Obviously, they only appear very late in the game, specifically in the Spirit Temple and in Ganon's castle. The first few times you have to fight one, you're placed in a room with just the Iron Knuckle itself, and are left to duke it out. But in Ganon's castle, you get the lovely opportunity to go up against two of them at once. From just this description, you're probably thinking these guys are pretty tough. Sadly, that's not actually the case.
First, they make sure to give you plenty of hearts. In the first several locales where you fight an Iron Knuckle, there are large pillars in the room. If you are any good at all at maneuvering Link around, you will find it easy to make the Iron Knuckle hit these pillars with its ax: this will release about 5 hearts. In addition, the Iron Knuckle itself will give you 3 or 4 hearts when you kill it.
Secondly, when you enter the room with the pair of Iron Knuckles, they don't attack you immediately. They'll wait until they are hit with your sword before attacking, meaning you can go up, hit just one of them, kill that one, collect the hearts, and then kill the second one. I thirst for (virtual) blood, so I always go up and fight them both at the same time, but if you're a big wuss this works pretty well.
And now, the big trump card: by the time you face an Iron Knuckle, you will have received Nayru's Love, a shield spell that protects you from all damage. So all you really have to do is walk into a room, see an Iron Knuckle, turn on the shield spell, and hack away at it with your weapon of choice.
Lastly, here is a technique to use when you are really really fucked. If you're out of magic (or don't have Nayru's Love), out of fairies, and nearly out of hearts, what do you do when faced with a big angry knight covered in half a ton of steel? You can't get close to him, or you're dead. Arrows don't work on him (not even magical ones). The answer: Bombchu. Yes indeed, those explosive mechanical rats will do you right every time! Just stand a long distance away and send a stream of Bombchu at the poor guy. He moves so slowly that you can easily fire three or four off and then run away to a different corner to begin your bombardment again.
An Iron Knuckle is also the focus of the best cut scene in the entire game (one of the neatest things I've ever seen in any video game), but I don't want to spoil it so I won't.
If you're wondering why I said it's too bad they're not so tough to defeat, it's because I rather enjoy the hands-on combat portions of the Zelda games, as well as the puzzle-solving. Update: As you could imagine, then, I thought Master Quest was a total blast.