Rock moles and xorns can eat any item made of iron, copper, silver, gold, mithril, or some other metal. Rust monsters can only eat iron objects, because the others are not subject to rust.
Iron that is not rustproof is considered a treat to pet metallivores. They are fairly easy to keep fed because most monsters that you defeat will drop metal armor or weapons. A pile of darts is a tasty meal for a pet rock mole, and a rust monster will happily eat all those pairs of iron shoes that are cluttering up the Gnomish mines. Metallivores can also eat food tins.