Monsters just spontaneously appear at certain points on the ground. One consequence of this is that you should never stick around at a spot after you kill something. This buys you enough distance that, if something spawns again quickly, you re far enough away to select it and fight it. It s pretty annoying when a new monster spawns right on top of you and you have to spin around, select it, and start fighting, all while it s whacking at you. You get a variety of monsters of approximately the same level appearing there.
Campfires are fairly common too.
If you see a campfire, either one that already exists or a new one appearing at a spawning point, be circumspect. These are often a sign that this spawning point is about to become Monster Central Station, with two or sometimes even three monsters spawning there simultaneously in the near future. Another thing you see quite a lot, usually associated with a kind of monster called a Shreth, is a rotting corpse lying on the ground. A lot of new players associate these with other players who ve lost a fight, but they re actually the outdoor equivalent of treasure chests. Once you dispatch the monsters guarding one, double-clicking the corpse will show you what it was carrying.