Ice Bat

They spawn through the ice caves before Hardmode, with nothing summoning them. The back half of that sentence is the useful part: once the Wall of Flesh goes down they become extremely rare, so the Bat Bat, the Depth Meter and the banner are all far easier to collect beforehand. Farm them on the way down, not on the way back.

Chilled slows you, which is a nuisance. Frozen locks you in place, which is how you die, because an ice cavern rarely holds only one thing. The freeze is uncommon per hit, but these come in numbers and a long cave run gives it plenty of chances. A Hand Warmer cancels both outright and drops the enemy to background noise.

It is bigger and hits harder than the Cave Bat you learned on, and it flies in the same lurching arc that sends swords through empty air. A gun or a bow settles it quickly. Melee only characters at this stage tend to eat two or three hits per bat. One small mercy: unlike most bats it cannot inflict Feral Bite on Expert.