Serpents are reguarded as poisonous and downright evil (except to those who have pet snakes, but I think those people are evil too..), thus a good personification of him. He could have just desended from heaven as an angel, which I don't understand why he didn't, but eh. Satan's a retard anyhow.
To CrAz3D, at that time, snakes did have legs.
Quoth the bible!I think snakes could talk as well, because.. hey, why not? But even if they couldn't, Satan (being an angel) could simply shapeshift (yay, druids!) into a snake and talk anyhow (which he did).
For you all,
Genesis chapter 3.
reports of snakes communcating with a person?
Parselmouth?