Flocko

They only exist inside the Frost Moon, and not from its start either. They begin arriving around the eleventh wave, which means you have to be past Plantera and well into a good night before you see one. They come in alongside the Ice Queen, and that timing is the problem: every second spent swatting spinning snowflakes is a second not spent on the boss you need dead before dawn.

They fly in a straight spin rather than tracking cleverly, so they arrive in a line and can be cut down as a group. Their defense is among the lowest of any hardmode enemy, so even weak area damage handles them. Anything that covers a wide arc, a spinning melee weapon or a broad spell, thins them without costing you time on the queen. Kills now feed the wave counter as well, so clearing them pushes the event along instead of being pure distraction.

Flock is the powdered fiber sprayed onto Christmas trees to fake snow, and the word sits close to the French and German words for snowflake. The sprite shows up in the artwork for reaching the fifteenth wave.