(Originally found at http://dngnsndrgns.blogspot.co.uk/2013/08/each-week-ill-be-taking-crack-at.html)
Level 2 (Magical Beast)
HP: 32 AC: 18 PD: 16 MD: 12
Bite : +7 vs AC - 10 damage.
Natural even hit or miss : If the target is wearing heavy armor it is now Rusting
Brittle defense : When an enemy makes an attack against the Rust monster, if they roll a Natural 1-5 on the attack roll the weapon they made the attack with is now Rusting.
Rusting : Any item that is Rusting takes a cumulative -1 penalty (to a maximum of -5) for every time its applied. For weapons this affects damage and armor it affects AC. It's at the GM's discretion if this effect is permanent, lasts until the end of the battle or until a time at which the player can repair his equipment.
Consume : +7 vs. PD (One target with a Rusting item) - The item, or an item chosen at random if they have more than one is destroyed.
Its worth noting that Rust Monsters make great adversaries because they hit the players where it hurts, not the HP or even the wallet - their equipment. Nothing is as scary as losing that magical item to a wandering beast with no name - so always use with caution and perhaps offer an alternative if a magical weapon is completely destroyed. 4e handles this by allowing the players to harvest raw magical essence from the Rust Monsters corpse.