An Old English term meaning hag or goblin. In Germanic superstition it used to refer to the female love-phantom, which was conceptualized as a special type of "nightmare. It was regarded as a manifestation of a demonic living being, capable of seducing and tormenting the sleeper in his or her dream. As noted by the German classical scholar Wilhelm Heinrich Roscher (1845-1923), a certain analogy would seem to exist between the mare and the " succubus. The term mare is used in opposition to the term "mar, which refers to a love-phantom of the male sex.
References
Roscher, W.H. (1972). Ephialtes. A pathological-mythological treatise on the nightmare in classical antiquity. In: Pan and the nightmare. Translated by O'Brien, A.V. Edited by Hillman, J. Dallas, TX: Springfield Publications.
Dictionary of Hallucinations. J.D. Blom. 2010.