Ghosts, Hags, Incubus, Succubus, etc.
http://www.britannica.com/bcom/eb/article/2/0,5716,43212+1,00.html
Incubus
...demon in male form that seeks to have sexual intercourse with sleeping
women; the corresponding spirit in female form is called a succubus. In
medieval Europe, union with an incubus was supposed by some to result in
the birth of witches, demons, and deformed human offspring. The legendary
magician Merlin was said to have been fathered by an incubus. Parallels
exist in many cultures. The word incubus is derived from the Latin incubus
("nightmare") and incubare ("to lie upon, weigh upon, brood"). In modern
psychological usage, the term has been applied to the type of nightmare
that gives one the feeling of a heavy weight or oppression on the chest
and stomach.
http://www.britannica.com/bcom/eb/article/7/0,5716,39557+1,00.html
Hag
...in European folklore, an ugly and malicious old woman who practices
witchcraft, with or without supernatural powers; hags are often said to
be aligned with the devil or the dead. Sometimes appearing in the form
of a beautiful woman, a succubus is a hag believed to engage in sexual
intercourse with sleeping men, causing severe nightmares and leaving the
victim exhausted. Although viewed in most lore as the antithesis of fertility,
the hag is believed by some scholars to be a remnant of primitive nature
goddesses.
http://www.britannica.com/bcom/eb/article/7/0,5716,117517+4+109534,00.html
exerpt:
Discussion of sociosexual behaviour would be incomplete without some
note of the role it has played in ceremony and religion. While the major
religions of today are to varying degrees antisexual, many religions have
incorporated sexual behaviour into their rites and ceremonies. Human beings'
ancient and continuing interest in their own fertility and in that of food
plants and animals makes such a connection between sex and religion inevitable,
particularly among peoples with uncertain food supplies. In most religions
the deities were considered to have active sexual lives and sometimes took
a sexual interest in humans. In this regard it is noteworthy that in Christianity
sexual behaviour is absent in heaven and sexual proclivities are ascribed
only to evil supernatural beings: Satan, devils, incubi, and succubi (spirits
or demons who seek out sleeping humans for sexual intercourse).