At one time people believed nightmares were dangerous. Maybe some of them still are....I've had a few horrors in my time (but that's another story 
) The 'mare' was a real creature, which could be conjured up by magicians to harm others.
The Anglo Saxons believed that nightmares were caused 'by the influence of elves' - no, not the sweet Cate Blanchett ones from Lord of the Rings, but the unpleasant variety that populated the Saxon World.
In fact, the Anglo Saxons blamed elves for a lot more than just nightmares. Sudden unexplained illnesses were caused by Elves firing arrows at people and animals, the resultant sickness was called being 'elf shot.'
Finds of the tiny flint arrow heads from the Stone Age were believed to have once belonged to elves....
maybe they were right!
