Rust monsters are an ungodly aggravation. They don't do any physical damage, but they will rust your weapon and equipment pretty darn quick. The best way to deal with them is to either cancel them or to rustproof your equipment. Once you've done that, they're a piece of cake. If you haven't been able to rustproof your gear yet, take off any metallic equipment (or in the case of body armour, cover it up with a cloak). They do no damage so you can simply punch them to death while naked or wearing junk as long as there are no other monsters to bug you as well. A wielded unicorn horn also works well. Note they will eat daggers you fire at them but would like to re-use, so be wary; if they eat a rustproofed item they will spit it out rustprone.
They will eat a ball and chain (from punishment) off of your foot and rust other monsters metal items. Because of this, they make semi-useful pets if you don't want any more metal items.
Before NetHack 3.1.0, rust (and other forms of object erosion) did not exist as such. Rust monsters and other things that caused rust instead reduced the enchantment; in the case of rust monsters this could go as low as -2. Thus rust monsters in older versions behaved more as modern disenchanters, save that their attack was still subject to an object's susceptibility to rust.
These strange creatures live on a diet of metals. They can
turn a suit of armour into so much useless rusted scrap in no
time at all.