Hellbat

Ordinary Underworld spawns. Nothing summons them and nothing gates them beyond getting down there, so they are in the air before you have finished laying a floor to stand on.

Lava is scenery to them. A moat that stops everything else down there does not burn them, though they will not fly into it and will hover on the far side instead. The contact damage is not really the danger. The shove is, because almost everywhere you meet one has lava underneath it, and being knocked off a bridge mid jump costs far more than the hit did. Lay solid ground and something at your back before you start mining hellstone.

Unlike most of the game's bats it cannot leave you bleeding, so your regeneration keeps running after a hit. In a Zenith world they spill lava when they die, which makes killing one directly overhead a habit to break early.

The name plays on the old phrase about moving like a bat out of hell. The sprite sheet holds an unused frame of a bat hanging upside down, shared across the bat types, hinting that roosting was considered and dropped.