At long last, I have discovered the true origin of the ears of the gryphon. In various stories, I have described them as horselike or deerlike -- not the ears of a lion, and certainly not of an eagle. The answer comes from a book called Gods, Demons and Symbols of Ancient Mesopotamia.
The ancient Syrians and Babylonians created the gryphon as we know it, as well as lion-demons, snake-gryphons, bull-men, and other chimerical monsters. Most of these creatures were depicted with long, pointed ears -- specifically, the ears of the Mesopotamian wild ass or donkey. There were two reasons for this.
What would you think if you saw a character with the physical attributes of a jackass? Would you think him a dim-witted bumpkin, like Bottom from Midsummer Night's Dream? Perhaps a fat, lecherous drunkard, like the Bacchus of Disney's Fantasia? A rotten, spoiled delinquent, like the boys on Pleasure Island in Pinocchio? Perhaps other words come to mind: stubborn, thick headed, obnoxious? And there are the various other meanings of the word "ass" itself in modern times.
Well, asinine as it may sound, in ancient Mesopotamia, the donkey -- or wild ass rather -- brought to mind totally different connotations. The desert-dwelling jackass, the fastest animal known to the Syrians and Babylonians, symbolized powerful, untamed Nature as much as the lion or the eagle. "The inclusion of an element of the swift-footed wild ass along with aspects of the fierce lion in such demonic hybrids might have seemed an appropriate combination of wild animals."  The pointed ears bestowed upon the gryphon (and later the dragon) were meant as a compliment. (It also means that gryphons are a triple chimera, so to speak, of lion, eagle, and just a dash of jackass.)
The second reason for the donkey's ears comes from the evil Babylonian/Assyrian goddess Lamastu. Lamastu "was above the common run of 'evil' demons. Unlike such demons, who acted only on the commands of the gods, Lamastu practised evil apparently for its own sake." 
The gods of Mesopotamia were associated with certain animals. Lamastu's animal was the donkey, and she appeared as a hairy, lion-headed being with donkey's ears and bird talons. Thus the hybrid monsters and demons of the Mesopotamian peoples were given Lamastu's donkey ears to make them appear all the nastier. "When such creatures were copied in arts outside Mesopotamia the ears were generally altered to those of a lion, an interesting example of lack of 'understanding' of the Mesopotamian convention. However, in Greek art the griffin retained its long ears, and these passed into the iconography of medieval and modern European griffins and dragons." 
"To signify impossibility or incongruence," writes Jorge Luis Borges, "Virgil spoke of breeding horses with griffons. . . In time, the expression Jungentur jam grypes equis ('To cross griffons with horses') came to be proverbial."  The Hippogryph, which has the front half of a gryphon and the hindquarters of a horse, was created by Ludovico Ariosto in his epic Orlando Furioso as a joke in reference to this old saying. Ironically, the gryphons had a bit of the equine in them all along.
1. Black, Jeremy, and Anthony Green. Gods, Demons and Symbols of Ancient Mesopotamia. (Austin, TX: University of Texas Press, 1992), p. 71.
2. Ibid., p. 116.
3. Ibid., p. 71.
4. Borges, Jorge Luis, and Norman Thomas di Giovanni. Book of Imaginary Beings. (New York: Avon, 1969 ), p. 124.
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