- It may come from Movable Object, to indicate that these creatures have the power to move around.
- It may simply be short for Mobile, again referencing movement.
Either way, it can still be used to reference NPCs that do not actually move (killer plants, for example). It also still retains the normal definition as a large group of individuals, so you can have "a mob of mobs". This may lead to some confusion following orders of "you take the mob on the right". For bonus confusion points, in a game with mafia characters, you can take on a mob of Mob mobs. Even so, it seems to be a persistent piece of slang in the MMO world.