Goat of Mendes
The goat of Mendes was a temple goat utilized in an ancient cult in the city of Mendes, Greece. Supposedly, the goat copulated with priestesses in certain religious ceremonies.
The cult was still known for its bestiality (and may have even been practicing) at the inception of Christianity during the reign of the Roman Empire. The practices of the Mendes cult inspired the Christian association of goats with Satan and paganism in general. This idea was especially strong during the Middle Ages, but persists to modern times.
The pentagram, a symbol utilized by pagans and Satanists alike, is also sometimes referred to as the goat of Mendes or goat head of Mendes as a result.