There are floaters, dragons, kites, rockets, monsters, etc. The older you get the more of them you see. If they remove the vitreal cell inside your eye most of that goes away.
Up until recently that sort of thing couldn't be done so folks would continue seeing all sorts of things that were simply defects in their vision, or hunks of flesh floating around inside their eyeballs.
Great point. And since the primary "power" of demons is over the imagination, not the intellect or the will, they can have a field day with those blood clots in the eye, i.e., floaters, dragons, kites, rockets, monsters, etc. in a susceptible individual.