Eating a bat corpse is a bad idea. It stuns you for 30 turns, and the corpse itself is worth only 20 nutrition. Pets, however, are for some reason immune to stunning, and can easily eat bat corpses.
Bats grow up to be giant bats.
All B monsters have slightly erratic movement. If the monster decides to move, there is a 1 in 3 chance that the monster will fly in a random direction, rather than approach you. If you are fleeing from a bat, this might provide a moment of relief. If you stay in melee range, the monster will also stay in melee range and continue to attack you.
Interestingly, any bat, giant bat or vampire bat generated in Gehennom is made slightly faster than normal. Presumably because nothing moves like a "bat [on its way] out of hell".
Bats and giant bats both appear early in the game, and the giant bats are individually more dangerous. Bats normally appear in small groups.
The bats of NetHack stay within melee range, making them easy targets for many players.
Real life bats have echolocation, employing sound to discover the positions of things in the dark. NetHack does not implement echolocation, though the biodiversity patch does, at least if you polymorph into a bat.
A bat, flitting in the darkness outside, took the wrong turn
as it made its nightly rounds and came in through the window
which had been left healthfully open. It then proceeded to
circle the room in the aimless fat-headed fashion habitual
with bats, who are notoriously among the less intellectually
gifted of God's creatures. Show me a bat, says the old
proverb, and I will show you something that ought to be in
some kind of a home.
[ A Pelican at Blandings, by P. G. Wodehouse ]