Skeleton

Skeleton is the ranged half of the Overworld's night shift. It shoots from a distance, backs away when you close in, and catches fire when the sun reaches it unless it is under cover, in water, or wearing a helmet. That retreat is the whole fight: corner one, close the gap behind a raised shield, and it has nothing left, while chasing one across open ground just feeds it shots. Its arrows hurt other mobs too, so a stray hit into a crowd starts an argument you can stand back and watch.

Bones and arrows are the point of them. Bone meal drives crop, tree, and flower farms, and arrows are ammunition you never have to craft, which is why a skeleton farm sitting over a dungeon monster spawner is a common early farm. The skull is a different matter — it drops only when a charged creeper's blast kills one.

Cold turns them into strays. A Skeleton left standing in powder snow long enough freezes into a stray, which fires Arrows of Slowness, and that is a variant you can manufacture deliberately with a cauldron and a bucket of powder snow. Wither skeletons in Nether fortresses are a separate mob entirely, and their skulls are what summon the wither.

A skeleton horse is not a variant of this mob but its mount. During a thunderstorm a skeleton trap can spawn one already carrying a rider, and killing the rider leaves the horse behind, tamed and rideable — the only route to one in Survival.