|Icon of both D&D and my childhood.|
For anyone who would like a little more background on the Rust Monster in RPGs, its SRD entry can be found here. There's also a good entry for it on the Pathfinder Wiki, including some interesting tidbits regarding its anatomy and physiology.
In trying to apply some real-world thought to the Rust Monster's capabilities, there are two big things that stand out. (1) When it touches metal with its antennae, the metal rusts extremely rapidly, and (2) the Rust Monster then eats the rusted metal. Now, the Pathfinder entry linked above explains these aspects really nicely. Apparently, the Rust Monster's antennae produce an electromagnetic field which rapidly oxidizes metal. This may not be a huge stretch, as magnetism can apparently impact corrosion rate. Then, the monster somehow absorbs the heat energy produced by the rusting process. I guess this is part of "eating" the rust. In the realm of D&D, this is all very plausible.
There are definitely some other explanations we could look to, though. Rapid rusting is an interesting thought in itself. It's just rapid oxidation of the metal, and we see rapid oxidation of materials all the time; we call it burning. I like imagining some sort of enzyme produced by the Rust Monster's antennae that catalyzes the production of iron oxide, essentially "burning through" huge chunks of metal almost instantaneously.
As for rust consumption, there are some interesting examples in nature of bacteria using iron in respiration. For example, check out Geobacter metallireducens, which basically breathes rust, using it as an electron acceptor in metabolizing its food. I love the idea of the Rust Monster harboring some sort of symbiotic bacteria in its gut that require this rust to thrive. Maybe the monster then gets energy, or organic building blocks, from these symbionts.
Of course, I don't really know if I should expect a Rust Monster to consume foods other than the oxidized metals it prizes. Maybe rust is just an essential nutrient...? The next time I throw a Rust Monster into an adventure, I may have to explore its diet a bit more.