Take WoW or Runescape for an example. You have an incredibly large map, filled with cities and forests, each filled with people and monsters.
Monsters roam around an area at random that is
25x25 tiles big (ambiguous). This is done server-side, so every player than comes across this area sees the same location of the same monster, and when a player attacks/kills that monster; other players see it too.
How is this calculated without being a massively intensive task? If you have 500 different types of monsters, all of which have 5 maximum spawned, you'll be calculating 2500 different paths continuously.