Bats are common flying enemies. They follow the Bat AI, randomly flying around and bumping into things while trying to make contact with the player.
Pre Hardmode Bats:
Cave Bat: Common Cavern/Underground enemy
Ice Bat: Common Underground Snow enemy, inflicts Chilled and rarely Frozen on impact
Giant Bat: Common Cavern enemy, inflicts Confused on impact
Illuminant Bat: Common Underground Hallow enemy
Lava Bat: Common Underworld enemy