Rust Monster

The hide of these creatures varies in color from a yellowish tan underside to a rust-red upper back. A rust monster’s prehensile antennae can rust metals on contact. The typical rust monster measures 5 feet long and 3 feet high, weighing 200 pounds.
  A rust monster's touch transforms metal objects into rust (or verdigris, or other oxides as appropriate). The metal oxides created by this monster are its food; a substantial amount of metal dropped in its path may cause it to cease pursuit of metal-armored characters.