Murk is an old word for darkness. It came into English about seven hundred years ago, from the Old Norse myrkr.

A mare (not the horse) was something worse. In Old English, a mære was a creature believed to trouble people in their sleep, leaving them with a feeling of being smothered. That is the mare in nightmare. In Norway the word is mareritt, and in Denmark mareridt: the mare-ride. Germans say Nachtmahr, and the Dutch say nachtmerrie.

Only in the 1800s did nightmare come to mean a bad dream. Before that, it meant the creature itself.

So a Murkmare is a dark thing that visits in the night. Now you know.