Some games have monsters that look like treasure chests, or other types of reward. The player walks up to this hidden monster expecting a reward and ends up fighting for their life! In some games the player gets the reward they were looking for, after the battle, in other games it is just a surprise encounter. Considering how the combat works here; static monsters on the sidewalk do not sound like a good idea. But wisps are mobile and can travel through the encounter zone. Of course the wisp monsters would stay on the street and never walk on the. "sidewalk." They would probably move the same way as regular monsters as well. If you see a wisp floating straight down the street chances are it is a monster. If you see it crossing the street, moving diagonally, or on the sidewalk then it is a real wisp. It may be difficult to pass behind a wisp monster, when crossing he street, as they have no distinguishable front or back, unlike most monsters. I know people may find such a monster to annoying. After all if you are low on health or mana you don't want to be ambushed. It was just the only way I could see to implement effective, "mimic," or ambush monsters, in this game.
Already in-game. Two bosses in DS have them as minions. They are known as Protection Pox and Damage Pox. They are used by Mavra Flamewing and Sandor Spearcaller. I have a good memory. I am in ZF.