Consistent with the "-vore" terminology, this would be a creature that eats metal. I don't believe any actually exist on Earth.

More importantly, in NetHack, it is often useful to polymorph into a metallivore in order to devour certain magical items and gain their major properties as intrinsics. For example, eating a ring of teleport could give you teleportitis, while eating a ring of poison resistance might grant intrinsic poison resistance. The advantage is that you are free to wear other rings, as the effect is now permanent.

This technique does not work with a ring of slow digestion, ring of hunger, ring of regeneration, or other such rings. Also, each ring eaten has only a 1/3 chance of actually bestowing the intrinsic (the ring is lost either way).

Rings aside, metallivores can eat any other metal items, such as tins, weapons, armor, whatever. I imagine this could be useful if you are starving to death, but I've never heard of anyone who saved themselves at the zero-hour by polymorphing into a xorn.

NetHack metallivores include rust monsters, xorn, and rock moles.