Theriomorphy, the manifestation of a spirit as a beast, is associated in India, Egypt, and Mesopotamia with ambivalent deities; in other cultures, an animal appearance was ascribed exclusively to spirits of evil. Animals associated with evil were the pig, scorpion, crocodile, dog, jackal, cat, rat, toad, lizard, lion, serpent, and dragon. Of these the pig, cat, toad, dog, and serpent appear most frequently in the Judeo-Christian tradition. The goat form of the Devil derives primarily from the image of Pan. From such theriomorphic ancestors the Devil inherited his claws, cloven hooves, hairiness, huge phallus, wings, horns, and tail.
(Jeffrey Burton Russell, The Devil: Perceptions of Evil from Antiquity to Primitive Christianity [Ithaca and London: Cornell University Press, 1977], 254)
Keith Burgess-Jackson, J.D., Ph.D. 