Targum Pseudo-Jonathan on (Genesis 3:14) tells us the original serpent was not a crawling thing. God "brought the three unto judgment" — Adam, Eve, and the serpent — and pronounced the serpent's sentence.
"Upon thy belly thou shalt go, and thy feet shall be cut off, and thy skin thou shalt cast away once in seven years; and the poison of death shall be in thy mouth, and dust shalt thou eat all the days of thy life."
The serpent once had feet. He walked upright. The punishment literally severed them. He became a creature condemned to its belly, shedding skin every seven years as a reminder of what he had done, carrying the poison of the death he had brought into the world inside his own mouth. What he gave to Eve — death — he would now carry forever.
The Targumist makes the punishment fit the crime with eerie precision. The serpent's gift to humanity was mortality. From now on, mortality rides in his jaws.