Monsters with the Batty flag use the bat AI. This makes them move somewhat randomly, and they tend to move away after attacking. As most batty monsters have a higher speed than the player, this makes hitting them after they hit you harder.
Batty monsters cannot push or be pushed by other monsters.
Be aware that undead forms of monsters which originally had the Batty flag will lose it, but will still have most of their original speed. While bat zombies and skeletons are still pushovers, undead harpies can be even more dangerous than the real thing; they may not hit as hard, but they hit far more often, as they waste fewer turns on movement.
If you have problems with a batty monster's move, attack, and move away tactics, move away from the monster yourself. It will then have to move and attack to hit you, finishing its turn next to you, allowing you to hit back.