Skeleton

The basic Skeleton is a Cavern walker with no ranged attack in Classic: it paces toward you, climbs small ledges and does its damage by touching you. What separates it from the slimes and bats around it is that it can work a door. That only happens during a Blood Moon, or occasionally in a Graveyard, so an ordinary Wooden Door on a cave base holds most nights and fails on exactly the night you need it. A Tall Gate is not a fix either, and there is a nasty detail attached: while a regular Skeleton is pushing at one, its contact damage ignores dodge effects, so a Black Belt or Brain of Confusion will not save you from standing on the wrong side of it.

In Expert some skeletons start throwing bones once they are close, which changes their role completely. In Classic you beat them by getting above them on a platform. In Expert that platform is now the worst place to stand, because the thrown bone arcs and the skeleton no longer has to reach you to hurt you. The throwing variant is not exclusive to Expert though: a Bone Skeleton Statue produces them on any difficulty.

A wired Skeleton Statue handles the ordinary version, which makes the Skeleton Banner and the early gear they carry, the Bone Sword, the Ancient Iron Helmet and the Skull, farmable without wandering the caves. Every skinned variant across the biomes and events counts as a skeleton for the Carton of Milk roll, which is a nod to calcium and bone growth, so any of the fifty or so kinds can produce one.