The Rust Monster

''This curious creature is drawn to steel the way rats are drawn to corpses. Some have observed that it seems to resemble the monstrous arthropods of Chur Gor, a strange and distant desert land.''

CR3 Rust Monster - Slain by Siegfried HP27 Initiative+3 Speed40(8) AC18(T13,FF15) Bite+3/+3(rust+d3+d4acid) SQ Darkvision, scent, corrosive secretions Saves+2+4+5 Abilities10,17,13,2,13,8

The rust monster's corrosive secretions destroy mundane metal armour worn by those it bites, and have a 50% chance to destroy mundane metal weapons that damage it.