” … The factor of fancy manifests itself in these handbooks of the Babylonian-Assyrian diviners in a form which is especially interesting, because of the explanation it affords for the widespread belief in antiquity in hybrid creatures such as satyrs, mermaids, fauns, harpies, sphinxes, winged serpents and the many fabulous monsters of mythology and folk-lore.
We have long lists of the young of animals having the features or parts of the body of another animal. Instead, however, of being recorded as a mere resemblance, an ewe giving birth to a lamb having a head which suggests that of a lion, or of a dog, an ass, of a fox or a gazelle, or ears or eyes which suggest those of another animal, it is stated that the ewe has given birth to a lion, dog, ass, fox gazelle, as the case may be.
In the same way, since it often happens that the face of an infant suggests a bird, a dog, a pig, a lamb, or what not, the fancied resemblance leads to the statement that a woman has given birth to the animal in question, which thus becomes an omen, the interpretation of which varies according to the ideas associated with the particular animal.
A lion suggests power and enlargement, and therefore a lamb or an infant with a lion-like face points to increase and prosperity in the land and to the growing strength of the ruler, and is also a favorable sign for the stall or house in which such a creature is born.
Favorable ideas, though of a different order, are associated with the lamb, pig, ox and ass, whereas with the dog as an unclean animal in the ancient as well as in the modern Orient, the association of ideas was unfavorable, and similarly with the serpent, wild cow and certain other animals, the interpretation refers to some misfortune, either of a public or private character, and occasionally of both.
This feature of a fancied resemblance between one animal and another and between an infant and some animal was the starting-point which led, through the further play of the imagination, to the belief in hybrid creatures and all kinds of monstrosities. The case of an infant being born with feet united so as to suggest the tail of a fish is actually recorded in our lists of birth-signs, and from such an anomaly to the belief in mermaids and tritons, half human and half fish, is only a small step, rendered still more credible by the representation in art which convents the resemblance to a fish tail into a real tail.
Since we have the direct proof  of the spread of the Babylonian-Assyrian system of divination from birth-omens, as of the two other systems above discussed, to Asia Minor, Greece and Rome, there is every reason to believe that we are justified in tracing back to this system the belief in fabulous beings of all kinds, though it may of course be admitted that there are also other factors involved.
We find this belief in Babylonia and Assyria, where we encounter in the ancient art hippocentaurs as well as bulls and eagles with human faces, and in the Assyrian art the winged monsters with human faces and the bodies of bulls or winged human figures with eagle faces. The process once begun would naturally lead to all kinds of ramifications and combinations.”
Morris Jastrow, The Civilization of Babylonia and Assyria, 1915, pp. 263-6.