Like has been mentioned above, demons exist in many forms and are not always evil. They are however somewhat unpredictable and chaotic.
From my own experiences, demons exist and are very real, but can only really cause you the most harm if you believe of their existence.
That being said, most of us can agree that there are various forces and "spirits" in the universe that help govern our thought processes, and therefore they are at least partially a figment of the mind.
Hope this is helpful~Rom