The Gorge

The Gorge would be a hostile mob that would actively seek out metal and any metal based ore. the Gorge would also actively follow and target the player. For example, if the player mines a piece of iron next to the Gorge, the gorge will stop devouring the iron in the vicinity and go after the player. also, the Gorge, if players aren't careful, will devour more metal(iron and copper) and other metals even more. so time could be ticking. additionally, since this mob would be more of a challenge and hinder the player in terms of progression or getting multiple items, defeating the Gorge would give the player a reward or unique drop. The Gorge also, when attacked, would barf out lava, or motel metal, thus making the challenge harder. the Gorge wouldn't just spawn randomly but spawn a bit like the warden near the player or more so in the vicinity of the player or even both. Thus, making this a challenge. The Gorge would also actively seek out the player, once spotted, and the Gorge would even be able to take ingots form a players inventory as well as their fighting. Also, the Gorge, if the player isn't careful, can come to the surface and raid villages or their village chests of their iron or metal, thus becoming a problem on the surface. The way I'd envision the Gorge is some kind of metal automoton, but not a golem. i'd be hard to tell if its some kind of machine,  similar to the guardian, although he Gorge would definitely be some creature that you couldn't exactly define.