Medieval legend claims that demons, both male and female, sexually prey on human beings. The male demon is known as an incubus and the female is the succubus. They generally prey upon the victim when they are sleeping, though it has been reported that females have been attacked while fully lucid. One such event was covered in the book and subsequent movie, The Entity.
There are variations of this sexual demonic legend. In Zanzibar, what is known as the "Popo Bawa" generally preys on men and only in their own beds. In the Chilo‚ Province of Chile, a wretched little dwarf, known as El Trauco, woos young naive women and then seduces them. Ịn Hungary, a Liderc is a demonic sexual predator that flies under the cover of darkness and will appear as a wispy apparition or a fiery light.
Any one of the above succubus' can be blamed for unexpected or unwanted pregnancies, especially in unmarried women, though one can imagine this might be a convenient fabrication to get out of unsavory rumors!
Some confuse the incubus with the legendary "Old Hag" syndrome but it is not. The Old Hag episode is usually restricted to a feeling of great pressure on the chest and not a ghostly sexual encounter. Another difference between the incubus/succubus encounter from the Old Hag is that the former is not always unpleasant while the Old Hag is mostly accompanied by a feeling of death, suffocation and just trying to survive.
Because the incubus and succubus are generally experienced during the sleep state or coming in and out of it, experts feel that it is an imaginary experience and not a real one, however, telling this to the person who has just gone through it, they find that hard to believe as it feels as real as intercourse itself. Who can say for certain if these events are real or imagined but until you've experienced yourself it's hard to determine or judge.