Most dictionaries state that the ethymological root of the English word nightmare stems from some kind of horse that haunts an innocent or not so innocent sleeper during their beauty rest.
However, the origin of this word could also date back to a female Germanic fable creature called „Nachtmahr“ that sneaked into the bedroom of her victim through keyholes or doorcracks haunting its prey by sitting on their chest causing them uneasy sleep.
These creatures are also seen as forerunners of the vampires, recently rediscovered by Hollywood’s money making machine.