The Mara is an evil 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 (e.g. the Swedish word "mardröm" literally meaning mara-dream, the Norwegian word "mareritt" and the Danish "Mareridt", both meaning Mare-ride or the Icelandic word "martröð" meaning mara-dreaming repeatedly). The mare is often similar to the mythical creatures succubus and incubus, and was likely inspired by sleep paralysis..
The Mara also serves as antagonist and arugably the main antagonist of Who Doctor Who where it is depicted as a snake