It is a very old belief, although buttressed by numerous tales of apparitions, that the Devil always appears with a cloven foot as a distinguishing mark. A suggested reason for such appearances is that the Evil One, having fallen to a level lower than any man is not be permitted to take the perfect human form, but must have some deformity such as the cloven foot.
Others think there is a more likely explanation, the early Christians superimposed on the concept of the Devil the image of the pagan goat-footed god Pan. A.G.H.
Source: 9.