The Rust Monster
From Dungeons and Dragons: Monster Manual I, the Rust Monster was a challenging and an unfair creature to “bump into ” underground.
“… they will go after ferrous metal in preference to copper, silver, etc.”
The Rust Monster is a frustrating creature for heroes. Not only can it strip you of your metal armor and weapons, but it is a level 5 monster. Dungeon Masters would begin introducing them to characters after they mature, surviving their first few adventures. Heroes proud, collecting enough gold to upgrade their armor and weapons go back underground to see it stripped away by a clumsy looking reptilian cricket.
It was solely a hazard. Never would a story would revolve around the lowly metal-eating monster. The Rust Monster was never popular. Although once, I remembered a hero took off all his armor and attacked the creature with nothing but a club!