The Desert Colossus

Yes, because he dresses like one, his attacks are pretty much shield proof, and he moves slower than you. The only difference is that he uses a chain ball instead of an ax, showing he's at a different level than most. Also, his Dark Dragon Form would be impossible for a normal Iron Knuckle, so it shows he is far more powerful than the average one.  

Eh.  I see both Iron Knuckles and Dark Nuts as animated suits of armor, so they're both the same to me.  As for General Onox, I'm pretty sure there is something that can turn into a dragon inside that armor, so it doesn't qualify as a Iron Knuckle/Dark Nut for me.

Then what about Nabooru and the Dark Nuts from TWW? They were more than just armor.

Because of that, I think Onox is a very powerful Iron Knuckle.

Ah, yes, TWW.  TMC also.  Slight overlooks on my part.

Still, I don't see any major differences that would set Dark Nuts and Iron Knuckles apart.  Maybe Iron Knuckles are the female ones.  *shrugs*

Even so, there's the possibilty that Onox isn't either.  He could just be a guy wearing a lot of armor.  Anyone can wear a ton of metal, really.  So perhaps, as I do believe Evilslayer suggested, Iron Knuckles and Dark Nuts are just a tribe of people working for evil.  And Onox could be a member of that tribe.  I suppose it all depends on the story you want to write.  Because that's how I decide what theories to go with; I fit the theory to the storyline.

Well, when removing Iron Knuckle's armor, you'll notice that it has SKIN unless you're blind.

That was more or less what I said.  But without saying that I saw skin underneath the armor.  Of course, I don't recall ever seeing an Iron Knuckle that showed some skin (asides from Nabooru, but technically she's not an Iron Knuckle).

What did Navi say about Iron Knucle Nabooru?

There's something weird about this one.

Were her possibly exact words.