Commentary Purg VIII 40-42

On the basis of Sordello's description, the serpent sounds both real and threatening enough for Dante to be afraid, especially since he has not yet read the signs as well as he will eventually.  There is only one other occasion in the poem in which Dante has been gelato (chilled): when he looks upon Satan ([Inf XXXIV 22]).  This serpent is indeed Satan in his 'serpent suit' in the garden (Genesis 3:1 and passim).