Mobs are living, moving game entities. The term "mob" is short for "mobile".[1]
Spawning
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Mobs come to exist by spawning in various ways. Most will spawn naturally at intervals, upon entering an area (for instance entering or exiting a cave) dependent on factors such as day/night conditions and biome, often times in groups with mobs of the same variety.
Most mobs are hostile, but already by the time you reach the Desert, you'll probably have encountered the Critter, a harmless being that likes to eat machines. It flees if scared, but occasionally topples you or your vehicle in its retreat.