Maras

Originally a Norse and Swedish term for goblins who attacked sleepers, robbing them of their faculties so that when they awoke they were bereft of speech. Later they were English night riding demons, usually demale, who preyed upon men and gave them bad dreams, hence the word "nightmare" ('nattmara' in Swedish).

Sometimes, they were the souls of jilted girls who visited their lovers while they slept, stimulating them to nocturnal emissions and general restlessness.