Reading through many articles, Iron Knuckles are referred to as "large, heavily armored knights". Knights of which kingdom is unsaid, but the idea that they are corrupted knights is definitely plausible, but there is one piece of theory that may bebunk that.
Take a look at Nabooru from Ocarina of Time. She was captured by Koume and Kotake, and brainwashed enough to where she turns against Link before he defeats her. In Zelda II AoL, they are referred to as "iron warriors, chosen by the King of Hyrule".
However, there is one piece of interesting trivia in regards to the Iron Knuckles. In Ocarina of Time, if Link positions himself very carefully, the camera will clip into the insides of the helmet. And what do you know, every Iron Knuckle has a female Gerudo inside. Here is a picture below, taken from a
Zelda Informer post.
Just like Nabooru, they have a blank, wide eyed stare that is completely emotionless. It seems very possible that they are indeed female Gerudo taken hostage by Koume and Kotake to add to Ganondorf's army. Perhaps Iron Knuckles are dedicated supporters of Ganondorf and were able to take the form via magic. If you take a look at this picture below, you can see a feminine physique underneath the armor that strongly resembles a Gerudo.
Now, this could all be Nintendo's way of using the same character model for all Iron Knuckles. But it's an interesting thought.