Hi Doug,
Details differ according to each culture but essential concept remains the same. You can take Bible as a sample case. Satan is personified and picturized by a snake. Snake is a perfect symbol of ego, and by extension symbol of Satan because it can easily access any place, even in paradise; it is the carrier of poison which symbolizes the final outcome of sensual pleasures. It doesn’t get rooted if a person doesn’t continue to focus on egoistic desires; but gets stronger and stronger if one continues to nourish egoistic desires. This is symbolized by snake having seven heads in some cultures or as ten headed monster in other cultures.