Rust Monster

Rust monsters are denizens of the underground that feed on metals, both ferrous and precious, though they prefer ferrous metals. You won't see them outside (usually), and they will only appear near veins of their preferred meals, even if they're not exposed. In this way, having these creatures around might actually be somewhat helpful, as they can lead you to iron and gold veins you might've otherwise missed. Also, like most other hostile mobs, rust monsters do not spawn in well-lit areas.

These creatures are not particularly interested in you, unless you happen to be wearing, wielding, or just carrying metal somewhere on your person, at which point they will attack you viciously (or if you attack them first). Not only do they attack you, but they will also do severe damage to iron or gold items on your person - and if you hit them with an iron or gold weapon or tool, it will damage that item incredibly more than it would normally.

If you get caught in a situation with a rust monster and would prefer to retreat than engage in combat, you can drop items it likes to eat to draw its attention away from your metallic hide. Dropping an iron ingot might not be fun, but if it saves you oodles of durability on your iron armor and weapons, it might be a worthwhile sacrifice. Of course, once you attack the rust monster yourself, it suddenly is more interested in making you dead than eating whatever you happen to drop... and, of course, once you're dead, you drop everything in your inventory. So the rust monster will gleefully consume every last bit of metal you had on you before you manage to get back to where you died.

Origin

Rust monsters are classic, iconic low-level monsters in Dungeons & Dragons whose current game stats can be found here.