When the damned things pop up into our perception, at many varied places over time, because they felt like it and as fallen angels they were originally created by God with independent volition. Go attempt to explain physical phenomenon such as a chest of drawers moving on its own across the floor or knickknacks being tossed about without proximate cause. Rare events, but nonetheless very obviously discernible when they occur.
You've missed the point in my question. Let's start again with just one question. When did the concept "demon" originate in history?
I'd guess that the mental state associated with "demonic possesion" has been with mankind for countless milleniums, but when did that state become associated with the devil?
The Devil -- there's another concept that must have some history, too. The Bible, perhaps?