The original rust monster was a rather goofy-looking creature, which legend tells us was inspired by a coin-machine toy. Having grown up in Wisconsin, and having seen the sort of toy that might have inspired the rust monster first hand, I am in no position to play “Myth Busters” here!
However, the rust monster is sometimes seen as nothing more than a “gotcha” monster. In fact, I have had discussions in which it was suggested that the rust monster would be better statted up as a “hazard” (ala Wizards of the Coast-style Dungeons & Dragons). This is a position that I reject utterly.
Rust monsters are not simply mobile hazards that do nothing more than leap out of the dark, allowing the GM to cackle maniacally as the fighter’s armour dissolves into a reddish-brown pile. No. They are creatures that can make sense within the fantastic milieus of D&D, and that can add depth to the campaign while adding interest to the dungeon.
Early rust monsters had no effective attacks apart from turning metals into rust, and so were ideal creatures for dwarves to harness with leather and wood, using them to locate rich veins while feeding them with lead and other base metals. Because rust monsters can turn any metal into rust, even non-ferrous metals.
This last ability may be of interest to sages, wizards, and other folk who craft magic items or spells. Indeed, the rust monster’s ability to detect metals may be important to the creation of certain wands, potions, and the abilities of some intelligent weapons.
By the 2nd Edition, rust monsters could defend themselves with a nasty bite. Thus, those same dwarves now had clubs and/or whips to keep the rust monsters in line. The dwarves would sometimes make use of bits of jagged glass braided into the leather of their whips, both to impress the rust monsters more, and to use against other creatures of the endless dark.
Of course, the dwarves also kept their rust monsters on long leashes, which they held partially coiled to limit the creature’s movements. When faced by an enemy, the leash could be extended, so that the enemy would have to deal with the rust monsters while the dwarves sent runners for archers. And, of course, any opponent deprived of armour and weapons would be quickly met with club and whip.
In early dungeoneering, taking off one’s armour to deal with the rust monster seemed to be an obvious thing to do. Yet, that assumes that the character will be free to don it when the rust monster is no more. Indeed, that assumes that no creature is watching, waiting for the character to do just that. Because other creatures have learned from their encounters with the dwarves, and they do not all mean adventures well!
It also assumes that the rust monster, like those of early Dungeons & Dragons, has no nasty bite. As that same early game encouraged creative refereeing, this wasn’t always a safe assumption even then.
The rust monster presented in 4th Edition is an anaemic version of its previous incarnations, whose ability to rust metals is strangely subject to reversing itself. Strangely enough, “Essence of 4e Rust Monster” may well be a component in rituals to mend items.
But it still isn’t just a “gotcha” monster. Which should come as no surprise to the clever GM, as it never was before.