On the track of Cerberus

The most ancient prototype of Milton's Sin is probably Echidna -- half woman, half serpent, mother of Cerberus and a boatload of other notorious monsters (whose father usually was Typhaon):

In the most ancient layers of Greek mythology, Echidna (Greek: Ἔχιδνα, ekhis, ἔχις, meaning "she viper") was called the "Mother of All Monsters". Echidna was described by Hesiod as a female monster spawned in a cave, who mothered with her mate Typhoeus (or Typhon) every major horrible monster in the Greek myths,

the goddess fierce Echidna who is half a nymph with glancing eyes and fair cheeks, and half again a huge snake,^[1] great and awful, with speckled skin, eating raw flesh beneath the secret parts of the holy earth. And there she has a cave deep down under a hollow rock far from the deathless gods and mortal men. There, then, did the gods appoint her a glorious house to dwell in: and she keeps guard in Arima beneath the earth, grim Echidna, a nymph who dies not nor grows old all her days.^[2]

Usually considered an offspring of Tartarus and Gaia, or of Ceto and Phorcys (according to Hesiod) or of Chrysaor and the naiad Callirhoe, or Peiras and Styx (according to Pausanias, who did not know who Peiras was aside from her father), her face and torso of a beautiful woman was depicted as winged in archaic vase-paintings, but always with the body of a serpent or having two serpent's tails.^[3]. She is also sometimes described as Karl Kerenyi noted an archaic vase-painting with a pair of echidnas performing sacred rites in a vineyard, while on the opposite side of the vessel, goats were attacking the vines:^[4] thus chthonic Echidnae are presented as protectors of the vineyard.

 In Hesiod, Cerberos had more heads:

Men say that Typhaon the terrible, outrageous and lawless, was joined in love to her, the maid with glancing eyes. So she conceived and brought forth fierce offspring; first she bore Orthus the hound of Geryones, [310] and then again she bore a second, a monster not to be overcome and that may not be described, Cerberus who eats raw flesh, the brazen-voiced hound of Hades, fifty-headed, relentless and strong. And again she bore a third, the evil-minded Hydra of Lerna, whom the goddess, white-armed Hera nourished, [315] being angry beyond measure with the mighty Heracles. And her Heracles, the son of Zeus, of the house of Amphitryon, together with warlike Iolaus, destroyed with the unpitying sword through the plans of Athena the spoil driver. She was the mother of Chimaera who breathed raging fire, [320] a creature fearful, great, swift footed and strong, who had three heads, one of a grim-eyed lion, another of a goat, and another of a snake, a fierce dragon; in her forepart she was a lion; in her hinderpart, a dragon; and in her middle, a goat, breathing forth a fearful blast of blazing fire. [325] Her did Pegasus and noble Bellerophon slay; but Echidna was subject in love to Orthus and brought forth the deadly Sphinx which destroyed the Cadmeans, and the Nemean lion, which Hera, the good wife of Zeus, brought up and made to haunt the hills of Nemea, a plague to men. [330] There he preyed upon the tribes of her own people and had power over Tretus of Nemea and Apesas: yet the strength of stout Heracles overcame him. And Ceto was joined in love to Phorcys and bore her youngest, the awful snake who guards [335] the apples all of gold in the secret places of the dark earth at its great bounds. This is the offspring of Ceto and Phorcys. link

Herakles, Kerberos and Eurystheus