Rust Monster

Most dwarves would rather face a squad of orcs than confront a single rust monster. These strange, normally docile creatures corrode ferrous metals, then gobble up the rust they create. In doing so, they have ruined the armor, shields, and weapons of countless adventurers.
A rust monster's body is covered in thick, lumpy armor, its long tail ends in a bony protrusion, and two feathery antennae sprout from its insectile head.
Underground Scavengers. Rust monsters roam subterranean passages in search of ferrous metals such as iron, steel, adamantine, and mithral to consume. They ignore creatures not carrying such metals, but can become aggressive toward those bearing steel weapons and armor. A rust monster can smell its food at a distance, immediately dashing toward the scent's source to corrode and consume the object.
A rust monster doesn't care if the rust it consumes comes from a spike or a sword. Adventurers can distract the creature by dropping ferrous objects behind them.
Subterranean Wanderers. Rust monsters are rarely found in large numbers, preferring to hunt alone or in small groups. They meander along tunnels, moving from cave to cave in their tireless search for ferrous metals to consume. Their wanderings often bring them into contact with other Underdark denizens that find them harmless or unappetizing. Thus, rust monsters may be found in close proximity to other subterranean monsters. If they are well treated and well fed, they can also become friendly companions or pets.