Rust Monster Ecology

So it's a long-known problem that rust monsters, which eat metal, are difficult to justify in an ecosystem containing, say, locks, iron doors, and so on.

Lore Sjöberg: Speaks With Monsters

Possible partial solution: rust monsters don't actually eat rusted metal; that's an assumption that in-character scholars make which is not entirely accurate. Instead, the rust monster's antennae deliver a catalyst that induces rapid oxidization (i.e., rusting), and the rust monster actually feeds on the energy released by this oxidization reaction. (Mumbo-jumbo about the catalyst is magical and that's why it can rust kinds of metals that normally don't rust.)

So if a door or a lock is already rusted all to heck, it's no good for the rust monster to feed on, unlike, say, combat-ready weapons and armor. (Allowing for metal objects in the dungeon to exist at a very specific level of rusted, not fresh enough for the rust monster to eat but not rusted enough to be inoperable.)

This in turn suggests the possibility of a related, perhaps more powerful species of monster, one that induces oxidization not in iron, but in carbon-based (i.e., flammable) materials. Which is to say, it sets you on fire and eats the resulting heat.