In roguelike games, the rust monster is a fast-moving creature that hits you with a rust attack. It will rust any iron-based armor you are wearing, and if you hit it with an iron-based weapon it will rust that as well. The rust monster does no hit damage, but it is intensely annoying to encounter one suddenly before you can take off your armor, and to watch your armor class plunge. Even more annoying is a rust monster in conjunction with another dangerous creature.

Here's a monster that will thrill you,
'Cause it doesn't try to kill you.
What a friendly little charmer--
Hope you're fond of orange armor!

--The Roguelet's ABC