Ambient entities

Ambient entities are objects that appear and move around in the background, and cannot be interacted with in any way. They are displayed depending on the time of day, biome, and weather. While some can be seen regularly, others are only displayed rarely. Clouds can be considered background entities although they are not affected by biomes. Ambient entities' motion speeds up with time.

Rare clouds
Every time a cloud is displayed, there is a chance it will instead be one of these rare clouds.

Trivia

 * The bats more accurately represent the bats from the Bat Scepter rather than the bat enemy.