In his sermon preached on September 28, 1958 (which can be read here), William Branham states, "...he was the smartest thing there was, and the more like the human being than anything else that was on the field: closest to a human being. He was not a reptile. The curse made him a reptile." Again, this idea makes the most sense to me because the curse wouldn't have been a curse at all if the creature remained in the same condition.
In condordance with Branham's statement that the serpent was the smartest (trickiest) thing, Dictionary.com defines serpent among other things as "a wily, treacherous, or malicious person. The Devil, Satan." Also echoing this concept is Nelson's New Illustrated Bible Dictionary, which says that the serpent is "...often associated in the Bible with temptation, sin, and evil."
Artists definitely have their own opinion of how the "serpent" looked as he was tempting Eve. Was he an upright snake as seen below?
(a) something other than a slithering snake before he was cursed? Or
(b) a serpent before he was cursed, only now he his sentenced to that state forever with no chance of escaping the serpent state?
Perhaps you made a third option "c" which is "I can see it either way" or "I have another idea of what might have occurred." If you have another idea, PLEASE comment and ellaborate. I'd love to hear your responses and ideas. As for me, it makes more logical sense for the serpent to have been something other than a snake before he was cursed since the LORD God clearly put him on his belly.