We are given no description of the serpent prior to the Fall except that he was “crafty” (Gen 3:1). The curse upon the serpent to go on his belly and to eat dust (Gen 3:14) could be reasonably interpreted as saying that the serpent did not go on his belly before the Fall. If we assume that the post-Fall serpent is the ancestor of modern-day snakes (and if assume that Genesis 3 is literal) then it is plausible to the serpent had legs before the Fall and God removed its legs as part of the curse.
However this is not the only way to read the passage. Since snakes do not literally eat dirt (they usually eat other animals) we might interpret the curse as indicating the humiliation of the serpent. Eating dust and going on its belly may simply mean that the serpent will be forever in humiliation.