Sui generis
In sociology, the "sui generis" is what has been externalized, then internalized in the overall public and becomes a part of society that simply exists in its construct. It is not something that is not thought to have been created, because it is imbedded in everyone's way of thinking and being. Instances include the idea of love, or going to school, or clothing belonging to a specific gender. These examples are "sui generis" for they simply exist in society and are widely accepted without thoughts of where they come from or how they were created.
Succubus
According to the "Malleus Maleficarum", or "Witches' Hammer", written by Heinrich Kramer (Institoris) in 1486, succubi collect semen from men they seduce. Incubi, or male demons, then use the semen to impregnate human females, thus explaining how demons could apparently sire children despite the traditional belief that they were incapable of reproduction. Children so begotten – cambions – were supposed to be those that were born deformed, or more susceptible to supernatural influences. While the book does not address why a human female impregnated with the semen of a human male would not produce regular human offspring, an explanation could be that the semen is altered before being transferred to the female host. However in some lore, the child is born deformed because the conception was unnatural.