The serpent in Paradise is a fascinating thing. I found a picture once, from an illustration of an 11th Century (or thereabouts) Saxon copy of Genesis. The serpent was pictured as a pretty serpentine creature with legs and wings. The artist had obviously taken the curse God placed on the serpent as being the removal of its legs and wings, so it and its descendents were forced to crawl from then on.
The relationship with dragons and angels is ancient, and is bound up in apocrypha like the Book of Enoch. Many ancient cultures imagined gardens with treasures, guarded by serpents called "watchers." (Greek drakones, dragons). Enoch has fallen angels called "watchers" though the Greek translations use gregoroi for "watcher." The Hebrew word sharaph is etymologically related to Semitic words for serpent and is translated both as one of an order of angels (Seraphim) and as fiery serpent (in Numbers 21:6-9) so we can figure the dual meaning was intact when the Septuagint was made (which uses ophis, meaning snake, in Numbers). Every Greek speaker should have been familiar with the myths in his own culture of snakes being drakones, so the snake/watcher/dragon relation should have been obvious. The Hebrew speaker should have been familiar with the serpent/angel connection, so the Satan as a fallen angel concept should not have been totally outrageous. Of course, Paradise Lost is the "modern angelic apocryphon" through which we all learn so much of our non-Biblical stories of Satan, but none of those ideas were new to Milton. It was not first proposed in Revelation, either. It had been around awhile.
