Bat

Bats are passive mobs that live mostly in caves. They will have three heart health. They won't drop anything.

SPAWNING
They should spawn anywhere at a light level of 4 or less on opaque blocks below layer 63

BEHAVIOR
It should be same as PC ones.

Bats sleep during day and becomes active at night. A Bat will hang upside down at block, but once when Player approach, it will fly away. Bats can't hang at transparent or non-soild blocks.

Bats are also 3rd smallest mob in the game (4th at PC)

Bats will also squeak (similar to their real life function of echolocation)

Althrough it's passive mob, they should despawn after going too far away like hostile mobs.

They don't recognize lava as obstruction, so Bats can fly into it and catch on fire.