Undead Miner

They live in the Cavern layer and they announce themselves. The helmet each one wears actually emits light, so in practice you see a wandering glow long before you see the enemy. That works for you twice over, since the same light sometimes leaks through stone and marks a sealed pocket of cave you would have mined straight past, the way a blooming Blinkroot does. A Lifeform Analyzer will also call one out by name, which turns a deliberate hunt into something better than wandering and hoping.

It walks at you and that is the entire plan. Plain ground AI means a short drop, a ledge or a gap in your platform stops it dead, and it has no ranged option to fall back on. The one wrinkle is doors. It can open them, but only during a Blood Moon or inside a Graveyard, so on an ordinary night a closed door holds.

It is the only source of the Bone Pickaxe and of the mining shirt and pants. There is no recipe and no second drop table to fall back on, so wanting any of those means going down to the caverns specifically. The Mining Helmet is the exception, since the Merchant stocks it and buying one takes a minute rather than an afternoon. Treat the helmet as a bonus that shows up while you farm the rest.

Bombs come off them often enough that a session down there leaves you with a working stack, useful early for cracking open ore pockets and cutting shafts. Steak turns up occasionally too, which is a better food buff than most of what is available at that point in a run.