A relic of a more superstitious age, sometimes a stone is found hanging from a beam or otherwise hidden in old houses, forgotten by all for centuries. This stone, called a Dobbie Stone or Hag Stone, always has a natural hole in it, thought to represent the all-seeing eye and ward off unwanted supernatural incursions. They were also hung in stables and animal byres to prevent the sudden onset of sickness and weakness now explained by viral or bacterial infection but formerly blamed on the Devil, the Faery or witchcraft. A horse that had been 'hag-ridden' in the night would all too often die in the following days, and it is thought that the term 'nightmare' comes from the same belief.
Similarly, in the house, a resident elemental called a Boggart, Boggle or Bogie was often to be found. From this we have the 'bogie-man' and 'mind-boggling.' They were considered responsible for all the phenomena which in the 20th century became the work of the poltergeist. They could make noises, move furniture, throw objects, tip sleeping people from their beds, curdle milk and cause all manner of chaos. Means were often taken to deter them, such as Dobbie Stones, rowan or hazel wreaths or crystals. But conversely, if treated with respect, a Boggart could perform helpful tasks, such as waking occupants in the case of fire.
Next time you have a nightmare, think of the night-hag who was hovering unseen above your sleeping form that night.