sitting there clothed and in his right mind.
And they were seized with fear.
-- Mk 5:1-20
Satan or the devil and the other demons are fallen angels who have freely refused to serve God and his plan. Their choice against God is definitive. They try to associate man in their revolt against God.
-- CCC 414
Art: Ivory pyxis with the Healing of the Demoniac, Carolingian, early 9th century AD. From Aix-la-Chapelle (Aachen, Germany), court school of Charlemagne, British Museum.