Minecraft wiki entry for Bat, associated text: According to Jeb, the bat is an example of an ambient mob – a mob that constantly spawns, similar to the squid, but also constantly despawns because when a player is not around it does not serve any purpose.[1]
It is impossible to attach a lead to a bat.
Between 20 October and 3 November, bats spawn more frequently as a small Easter Egg in homage to Halloween.