Everyone knows what the meaning of “night” is. As for “mare”, well, one of the meanings is a “female horse”. So a nightmare should be a female horse that appears in your dream. But generally we don’t dream about horses. Usually we have terrible dreams about falling off a building/mountain, or appearing for an exam when we are totally unprepared. I understand that in Old English the word “mare” meant an evil spirit. And since it was evil, it was assumed to be a “she” rather than a “he”! Once again showing how sexist this language is. It was believed that this evil spirit sat on people who were sleeping causing a feeling of suffocation.