That's not a Rust Monster, it's a Corrosion Creature, and if "Gazers" are fair game... :) Besides which, the appearance that TSR gave the Rust Monster was not original either. It was, along with the Bulette, based on little plastic monster toys that were common back then (and covered under design patents, not copyrights, thus TSR's ability to use the appearance thereof in a drawing). Even the sizes of the monsters is based on those toys' sizes as compared to 25mm lead figurines.

I'm the asshole. You wanna be a shit? You gotta go through ME.