Nutcracker

Nutcrackers belong to the Frost Moon and nowhere else. The world has to be in Hardmode with Plantera already beaten, the event has to be running, and they only start arriving once the moon has climbed a few waves in. There is no way to find one on an ordinary night.

Knock one down past roughly half its health and it does not die, it transforms. The second form spins on the spot like a dancer, then moves at several times the pace of the first and shrugs off knockback that used to hold it in place. A crowd that felt slow and easy to kite becomes a pack closing faster than you can walk backward. Plan for the switch: when the spinning starts, either commit to killing them or get off the ground.

Every one you drop feeds the wave score, so nutcrackers carry you toward the later waves where the good drops sit, and a kill counts for more of that score in the harder difficulties. They come in numbers, which makes them better fuel for a piercing weapon aimed along the arena floor than something to duel one at a time.