I had a brief stint working on the Ch'tauri (though they weren't called that at the time) while in the Visual Development department. I took a couple of passes of refining Justin Sweet's original designs and attempting to define the look of the armor & weapons and how they might integrate into the creature. Ultimately Ryan Meinerding developed the final look in Z-Brush.
I imagined the helmet as being a little deceiving, so that ultimately when the mask was ripped off you would find that the creature had air holes in the place of eyes, which would actually be to either side of the mouth. I also liked the idea of some kind of "light chainsaw" that would wrap around the forearm. Kind of the hedge trimmer of your nightmares.