Unicorn

The Hallow produces them, but only above ground and only on the right footing: hallowed grass, pearlstone, pearlsand or pink ice. The Underground Hallow does not spawn them at all, which is why a cavern trip through pink stone stays quiet while a walk across the surface does not. A Unicorn Statue will place them before Hardmode if you want the fight early, though statue spawns hand over nothing when they die.

It moves fast, clears roughly seven tiles of height and twenty of distance in a leap, and it charges rather than approaching. That combination means it arrives from offscreen on a line you never planned for. Worse, it barely reacts to being hit, so the knockback that normally buys you a second of breathing room does close to nothing. Standing still with a slow sword in early Hardmode is the classic way to lose most of a health bar to one.

• Water slows it enough that ordinary sword knockback starts landing again.
• A hole two tiles wide is too narrow for its body. Stand in it and hit upward while it fails to reach you.
• Confusion turns it around and sends it away, which is why the Dao of Pow punches above its weight here.
• Reach weapons let you fight from outside trample range. Boomerangs, spears and yoyos all work.
• A properly reforged Golden Shower can pin it in place, and the Clinger Staff pairs heavy knockback with Cursed Inferno.
• Bouncing on one from the slime mount kills it without you moving, and picks off any Pixies that join in. Do not try it after dark, when Gastropods can shoot you out of the air.
• A chainsaw or a very fast sword works too, since repeated small knockback keeps interrupting the charge.

Every kill gives a Unicorn Horn, and you will want a pile of them, so a Hallow trip early in Hardmode is worth making on purpose rather than by accident. The Blessed Apple is the rare one, and there is nowhere else left to get it since the other enemies that used to carry it no longer do. The mount it summons is among the better ground options available at exactly the point in a run where you are still meeting unicorns, which makes hunting for it less of a detour than it sounds.