The term mar comes from the Old English noun mare, which means hag or goblin. In Germanic superstition it was used to refer to the male love-phantom, 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 while he or she is dreaming. As noted by the German classical scholar Wilhelm Heinrich Roscher (1845-1923), a certain analogy would seem to exist between the mar and the " incubus. The term mar is used in opposition to the term " mare, which refers to a love-phantom of the female 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.