"There is, too, a very general rumor, which many have verified by their own experience, or which trustworthy persons who have heard the experience of others corroborate, that sylvans and fauns, who are commonly called ‘incubi,’ had often made wicked assaults upon women, and satisfied their lust upon them; and that certain devils, called Duses by the Gauls, are constantly attempting and effecting this impurity is so generally affirmed, that it were impudent to deny it."
Augustine, City of God, XV, 23
"Still if some are occasionally begotten from demons, it is not from the seed of such demons, nor from their assumed bodies, but from the seed of men taken for the purpose; as when the demon assumes first the form of a woman, and afterwards of a man; just as they take the seed of other things for other generating purposes, as Augustine says (De Trin. iii), so that the person born is not the child of a demon, but of a man."
Thomas Aquinas, Summa Theologica, I, 51, 3