Mare

A mare or nightmare is a spirit or goblin in Germanic folklore which rides on people's chests while they sleep, bringing on bad dreams (or "nightmares"). The mare is attested as early as in the Norse Ynglinga saga from the 13th century, but the belief itself is likely to be considerably older. As in English, the name appears in the word for "nightmare" in the Nordic languages. The mare is similar to the mythical creatures succubus and incubus, and was likely inspired by sleep paralysis.

The mara was also believed to "ride" horses, which left them exhausted and covered in sweat by the morning. She could also entangle the hair of the sleeping man or beast, resulting in "marelocks", called marflätor "mare-braids" or martovor "mare-tangles" in Swedish. The belief probably originated as an explanation to the Polish plait phenomena, a hair disease. Even trees could be ridden by the mara, resulting in branches being entangled. The undersized, twisted pine-trees growing on coastal rocks and on wet grounds are known in Sweden as martallar "mare-pines."