In Genesis it says satan was disguised as an "angel of light".
In Jasher, it says satan appeared 3x to Abraham in the likeness of 3 different old men.
In Jasher also, satan appears as an old man to Sarah, and again as another person to Isaac.
satan embodied Judas Iscariot at the last supper.
Are you getting it now?
satan doesn't show up as himself
he hides what he truly is, for if he didn't, where would be the deception?
Perhaps the book of Genesis refers to him as a serpent for this reason:
snakes tend to camouflage very well with their environment, so that if you're trouncing around the woods, let's say - and not paying attention- you could very well step on one; and when you get so close, of course you're vulnerable for a bite.