Bat

Bats are flying passive mobs that spawn in caves.

Spawning
Bats can spawn in the Overworld at a light level of 3 or less on opaque blocks below layer 63. Between October 20 and November 3, they only require a light level of 6 or less to spawn. On Superflat worlds, bats spawn below the top non-air layer, ignoring the usual layer-63 limitation.

Drops
Bats do not drop any items or experience when killed.

Behavior
Bats are very small, being less than half a block tall and wide. They will randomly squeak. They often fly around aimlessly, tending to favor the east, and will sometimes fly into lava and catch on fire.

Bats also will often hang upside down on the underside of a solid block when idle, though they cannot hang on transparent or non-solid blocks. If a player approaches it while hanging upside down, including if the player is invisible, they start to fly away from the player. If you break the block a bat is resting on, the bat will go into the flying state and fly away from you.

Although the bat is a passive mob, they still despawn when going too far away, similarly to ocelots and squids.

Data values
Bats have entity data associated with them that contain various properties of the mob. Their entity ID is.

Trivia

 * It is impossible to attach a lead to a bat.