Cleon
Legend
I wouldn't bother excluding invertebrates. If it can detect magnetic field so acutely it should also be able to detect electric fields too, so will be able to pick up the nerve-impulses of living creatures like an electric eel or duck-billed platypus can.
Besides, most of the invertebrates D&D players fight are giant ones that must be physiologically very different from regular-sized ones, so for all we know they may use iron-based blood (because they've been cross-bred by drow?
)Either that, or it's magic.
Basically, I'd just give it blindsight and note its blindsight also allows it to judge the type and weight of any metal it "sees".
e.g.
Metal Sense (Ex): A harrow's blindsight can precisely sense the weight, type and location of any metal within its 60 ft. range. It can even detect the traces of iron or copper in a living creature's blood. This metal sense can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt blocks it.
