One dream experience has collected demon names in every language: the sleeper wakes unable to move, feels a weight on the chest and a presence in the room, and is certain something is there. Latin named the crusher incubus; Germanic folklore the mara, the crushing spirit hiding inside the word nightmare; Newfoundland calls it the Old Hag and being ridden by her hagging. Sleep science knows the experience as sleep paralysis with hypnagogic hallucination, common, benign and cross culturally stable, which explains the universality of the demon reports without explaining away how convincing they are. Any entry on dream demons owes the reader that fact first.
The interpretive traditions handled the rest. Late antique and medieval dream theory, following Macrobius, sorted dreams into ranks and set the phantasm and the nightmare among the false and bodily kinds, while the church read demonic dreams as temptation requiring discernment rather than fear. Islamic interpretation similarly ranked the bad dream as from the adversary, to be left untold and not acted on.
The psychological schools brought the demon indoors: the Freudian line read demonic figures as the repressed returning in costume, and the Jungian as the shadow personified, the dreamer's own disowned material given horns precisely so it cannot be recognised as their own. Popular dictionaries compress to inner fears and guilt. The readings are the traditions'; the paralysed visitor at the bed's foot, at least, now has a physiology.