Cave Bat

Cave bats fill the cavern layer before Hardmode and they are the most common thing you will meet down there. Once Hardmode starts, giant bats take over most of the spawn slots and these become scarce, which matters if you still want anything off their drop table.

The damage is not the problem, the hitbox is. They are small and they fly in jerky, unpredictable arcs, which makes them awkward to lead with an arrow or a spear. A wide melee swing or anything that homes costs you far less health than trying to time a precise shot. They also cannot enter water, so a flooded pool or even a shallow channel is a free safe spot to stand in and swing from.

Feral Bite is what turns them from filler into a threat. It piles random debuffs on you and outlasts the fight itself, so one careless hit while spelunking can follow you halfway across a cave system. Clear them out properly rather than running past.

A Bat Statue removes the search entirely. Statue spawned bats still hand over their item drops as long as you land the kill yourself instead of letting lava or a dart trap do it, though they will not give coins. Do this while the world is still pre-Hardmode and they are the common bat. In Zenith and Remix worlds the Chain Knife is swapped off their table for the KO Cannon, which is worth checking before you spend an evening on it.