Thumper

Take an armadillo, give it ursanoid qualities, an ankylosauridae tail, and a pair of caustic acid glands under the tongue.

Thumpers have an appetite for in descending order; plant leaves and grass, metals, magicly active crystals (breeding season only), flesh (armor encased). Honey is also eaten by Thumpers, but mostly as a supliment and primarily in the fall. Meat is eaten only when it gets in the way as it were or where metals are scarce (Iron in the blood).

The Metals and Crystals are absorbed into the body and are primariy deposited in the carapace(For AD&D'ers and Everquest II pen and paper give them masterworked or mastercrafted armor +armor/mitigation). Both genders increase their disolved metal and crystal intake as breeding season approaches.

Basicly if I were to redesign the standard AD&D Rust-Eaters this is the way I'd go.