Bat

Bats are passive mobs that spawn naturally in caverns or other large enclosed spaces. They sleep during the day and become active at night. A bat will hang upside down when it is idle, but once the player approaches, the bat will fly away. Bats tend to fly incoherently and sometimes run into lava.

Trivia

 * Bats can spawn in a house if there is a large enough area for the game to think of it as a cave.
 * Bats are the only passive mob able to fly, and the only flying mob that spawns in the overworld.

Gallery
Fledermaus Chauve-souris Nietoperz Летучая мышь 蝙蝠