Lava Bat

It does not exist in a fresh world. The Underworld runs on Hellbats until you put down your first mechanical boss, and from that point this takes over as the stronger version. Anyone who was comfortable strip mining hellstone or building an arena down there before Hardmode notices at once, because the old routine of eating a couple of hits and carrying on stops holding up.

Same jittery, unaimable flight path as every other bat, which wastes shots from anything you have to lead. Homing weapons and wide melee arcs clear a group far more reliably than precision does. The one mercy is that despite being a bat it cannot give you Feral Bite, so unlike the jungle and cave varieties there is no debuff roll to plan around. The Bat Bat comes off it rarely enough to be something you notice while doing something else, never a reason to farm.

On that seed each one spills lava when it dies. A bat shot down over your platform dumps that lava onto whatever you were standing on, which turns routine clearing into a hazard you created yourself. Roof the area, or fight somewhere you do not mind losing.