The word "serpent" is a loaded one. Sure, it's a synonym for "snake", but the connotation is totally different. "Snake" is a mundane term to describe your basic scaly, legless, carnivorous reptile lacking ears and eyelids and with a penchant for swallowing its prey whole—but "serpent", now, ah, there's a term haloed in mystique and a-shadow in mystery, colored by hints of the supernatural, the religious, the occult.
The serpent is prevalent in religions, folktales, myths, legends, and fairytales across the world. These slithery not-really-beasties are tempters and guardians, gods and demigods, maleficent and beneficent. In contemporary society (yes, yes, U.S.-ethnocentrism fully included in that statement, thank you) the serpent most often gets booted across the line of malignity and is regarded with suspicion, condemned as deceitful, and feared as a symbol of evil.
But why should the snake and serpent attract such fascination? Well, for one thing, there's appearance. There are relatively few limbless creatures found the world over—and most of us find ourselves wondering at some point in our childhood, "How the hell does a snake slither anyway?" This reptile manages mobility despite a lack of arms, legs, fins, wings, pseudopods, tentacles or any other form of appendage. So they're creepy there because they're different, and when things are different, they tend to be scary, or unlikable, or odd, or otherwise negatively regarded. Snakes are obviously low to the ground and thus become associated with lowliness, and they tend to either crush their prey to death or inject them with deadly venom in order to kill them, not to mention the whole swallowing-them-whole thing which is downright disturbing to some people. The striking snake is an image of speed, and since striking snakes are usually the venomous ones, and some snakes are very venomous indeed, it becomes an image associated with the speed of death—and from something so innocuously low to the ground it's easily overlooked.
Snakes are also extremely common throughout the world, unlike platypuses, pandas, and penguins. So being extremely common, weird-looking, possibly extremely deadly, possibly exotically colored or prettily marked, it's no wonder that mythology and the human mind seized upon snakes as intriguing little buggers millennia ago and hasn't relinquished its death-grip since.
Surely the best-known serpent is the Serpent of the Garden of Eden, for "the serpent was more subtile than any beast of the field which the Lord God had made" (Genesis 3:1). This serpent is twined around the Tree of Knowledge and appears solely, it seems, with the intent of seducing Eve to
(Also, Satan/Lucifer/the Devil is frequently depicted throughout various periods of art as a dragon as well as a serpent. My AP Art History is long behind me, but a great many cathedrals possess statuary depicting various saints or Christ stepping on dragons and/or serpents, representing the conquest of evil. Also, slightly randomly, forked tongues have long been symbolic of deceit and malicious words, hence the term "to speak with a forked tongue" and how the Devil is said by some to have a forked tongue. I seem to recall having heard about tongue-splitting used as a form of torture or punishment meant for those who were caught lying—and a cursory Google search reveals that when Byzantine emperors were overthrown, yes, apparently their tongues were split as a form of torture. Ow. And generally speaking (pun halfway intended), cutting out of the tongue has been an ancient form of punishment in lots of cultures, usually for lying or slander or deceit or the like. Even more ow.)
In Greek mythology, a prominent serpent is the Python, the chthonic guardian of the Oracle of Delphi that was slain by Apollo. There's also the poison-blooded Hydra that grows two heads for every one cut off, slain by Heracles as one of his Twelve Tasks—though the Hydra is less specifically snaky being that it is often depicted as having limbs; the heads, though, are occasionally of the serpentine persuasion, and generally it seems agreed that the Hydra is serpenty or at least dragonish and reptilian. The fire-breathing Chimera is usually pictured as a beast with a lion's head and a goat's head and body and a snake for a tail. There's Echidna, the mother of all monsters (including the Hydra and Chimera), woman from the waist up and coiling thrashing serpent from the waist down; there's also Lamia, originally queen of Libya, who murdered her children and turned into a serpent from the waist down and had the power of prophecy and apparently the ability to temporarily yank her eyes out of her head. And of course we can't forget Medusa, one of the three Gorgons (or in some accounts she is the sole Gorgon), who has serpents springing from her head in place of hair and whose face could turn anyone to stone.
So women + snakes = a winning (evil) combo for the Greeks. But! There are also instances of snakes providing wisdom or otherwise being of aid to humans. According to some versions, blood taken from a Gorgon's left side was lethal poison whereas blood taken from her right side could return the dead back to the living. In some tales, Asclepius, son of Apollo and the most renowned mortal healer of Greek mythology, is given a vial of the Gorgon's life-giving blood which he uses to "cure" the dead (for which later Zeus smites him with a thunderbolt since Hades whined that the souls of the dead were his, his, his, and Ascelpius was stealing them). Asclepius also bore with him a staff twined around with a serpent that would whisper causes of patients' illnesses into his ear. (The Rod of Asclepius is apparently the original medical symbol and not to be confused with the Caduceus, which is a winged rod with two serpents wound about it, though even today it is still being used as a medical symbol.) Zeus eventually put Asclepius in the sky as a constellation, Ophiuchus or "the serpent-bearer". The healer and seer Melampus found a dead mother snake on the road and gave her a proper burial, for which he had his ears licked clean by her grateful offspring so that he could hear and understand the language of animals, enabling him to have all sorts of adventures and garner interesting information.
Moving on to other cultures and societies, there are the naga of Hinduism and Buddhism, which are deities taking the form of huge serpents ("naga" is apparently a transliteration of the Sanskrit word for "snake"). They sometimes appear as a mix of human and serpent and are preyed upon by the bird-god Garuda. And of course there are the naga people of Piers Anthony's Xanth novel series, able to shift forms: fully snake, fully human, or a combination of snake and human (human from the waist up or a human head on a snake's body). In Japanese folklore, serpents and snakes are generally symbolic of jealousy, for there are many tales of women transforming into serpents due to jealousy. (Interestingly, there are extremely few (if any) tales of men transforming into snakes, either on account of jealousy or otherwise. Demons, spirits, and ghosts? Yes. But snakes? No.) The Aztecs worshipped Quetzalcoatl, a feathered serpent, as a god of both sky and creation as well as a patron of knowledge. Another feathered serpent (possibly related/connected to Quetzalcoatl) is Avanyu, a water guardian and storm god of the Native American Tewa people, represented as a horned, feathered serpent. And in Chinese mythology and folktales there are apparently a number of stories featuring snakes, for which I got to essentially sit down for Story Time with Krispy, who recounted a few to me, including one about a snake that a boy raises up from a wee snaky, for which in gratitude it spits up a pearl that brings him wisdom, success, and general happiness in life, until he gives it away as a bribe and has the gall to ask the snake for another, whereupon it bites him to death; and another story about a giant lotus that appears every morning floating on a lake, which people sit on because a wandering monk tells them it's a holy lotus that will take them to the Pure Land—and which in the end turns out to be the mouth of a gigantic serpent that ate all those credulous superstitious people.
Ouroboros, the name generally now given to any depiction of a serpent holding its tail in its mouth, symbolizes eternity, time, cycles, round-and-round the merry-go-round and what have you. The gold ring the sorceress-type Aes Sedai women wear in Robert Jordan's Wheel of Time series takes this form. A variation of the snake-biting-own-tail is also prominently featured on the cover of the book that Bastian finds in Michael Ende's The Neverending Story, featuring two serpents (one silver, one gold) biting each other's tails; this symbol is also found throughout the story. In Norse mythology, Jormungandr, the World Serpent or Midgard Serpent, is a serpent that gets flung down into the ocean and eventually grows large enough to encircle the world and bite its tail.
In J.K. Rowling's Harry Potter, one of the four Hogwarts Houses is Slytherin (link contains some analysis and may contain mild spoilers up to book five-ish), named after former House-head Salazar Slytherin; its colors are green and silver, it's mascot is a serpent, and according to the article I linked to, J.K.R. purportedly once said somewhere or other that the House is loosely affiliated with the element of water. According to groundskeeper Hagrid, I paraphrase, "There are no wizards gone bad who weren't in Slytherin." Slytherins are widely touted to look out for Number One above all else, and are cunning ends-justify-the-means type of people—and of course provide 85% of the antagonism within the daily academic life of Hogwarts.
And of course there's Parseltongue, the ability to communicate with snakes; notably there are no other animal-communication talents mentioned within the scope of the Harry Potter world, thus singling out snakes as a mysterious (and malign) force. You don't see Voldemort wandering around with a cuddly big-eyed bunny familiar—oh no, he's got a frickin' huge killer serpent named Nagini. Sure, he may have his reasons considering his ties to Slytherin House and all, but in the end the fact remains that he's got an evil snake at his beck and call and is constantly associated with serpents.
There are also basilisks in the Harry Potter world. In European mythology, the basilisk is a creature with a power similar to that of the Greek Gorgon sisters whose gaze could turn humans to stone—accounts vary as to whether the basilisk's gaze kills at a glance or turns to stone. The purported king of serpents, the basilisk tends to be pictured several different ways: a gigantic serpent (fittingly), a cockerel with a snake's tail and teeth (similar to the cockatrice, itself another legendary beastie of the serpentine persuasion, though it's supposed to have wings, unlike the basilisk), and a multi-legged lizard. Basilisks can often be found as minor monsters within the various Final Fantasy games with the power to inflict "petrify" status upon your party members, turning them into stone. (This condition can usually be remedied through the use of several respective items, including the creatively named "remedy" and even more creatively named "soft".)
Speaking of Final Fantasy, a frequent ally/Summon-type monster is Leviathan, who usually takes the form of a gigantic blue serpent and deals tidal waves of usually water-based damage upon enemies. (In Final Fantasy IV, in fact, Leviathan is the King of the Land of Summons.) The term leviathan has its origins as a monstrous sea creature in the Old Testament; there seem to be a number of versions stating that originally God created two leviathans, a male and a female, but that He killed the female to prevent them from procreating and overrunning the world. Very considerate. In the Book of Job, though, Leviathan appears to be a normal ordinary everyday humdrum mundane beastie alongside goats and hawks.
And who can forget evil Grand Vizier Jafar of Disney's Aladdin, whose staff of power is shaped like a cobra. The Vizier himself later takes the form of a gigantic black cobra in order to crush the life out of hapless Aladdin (rather odd that he doesn't just bite Aladdin's head off, swallow him whole, or pump him full of venom, but then again, I guess that wouldn't be acceptable in a Disney movie). Going along with other Disney portrayals of snakes, there's Sir Hiss of Robin Hood, whose power to hypnotize with his eyes is similar to the powers of Medusa and the basilisk; mesmerizing/hypnosis by means of the snake's eyes is also a common theme in general snake myths. Sir Hiss is King Richard's advisor and used his hypnotic powers to send Prince John on a crusade in the first place, thereby exemplifying Sir Hiss's deceitful nature and—you know what, it's a Disney movie and fairly self-explanatory. Same with Disney's version of The Jungle Book with the python Kaa hypnotizing Mowgli and deceiving various characters because he wants to eat the so-called man-cub. (And for the record, The Jungle Book was produced in 1967 while Robin Hood came out in 1973; Kaa and Sir Hiss look basically the same and the googly-stripy eyes they have when hypnotizing are pretty much the same too. Seems like Disney got lazy with their character designing.)
From this mish-mash of mythology and pop culture, we can determine that snakes have gotten a pretty bad rap as the centuries have gone along, and really, things haven't gotten much better. We can also determine that there seems some sort of malign link between serpents and women, if the Greeks, Japanese, and the Bible have anything to say of the matter, to say nothing of the other mythologies and folklore I'm less familiar with. I'm sure there's probably some psychologist or literary analyst out there (not even necessarily Freudian) who says that the serpent is a phallic symbol and something or other about women donning masculine characteristics and transforming monstrously as a physical result and representation of the unnaturalness of women acting like men—but I've said enough already right there. After all, to literary analysts, everything is a phallic symbol. I have this on good authority. More than one of my literature professors has said so. (They really have. In almost exactly those words.)
At least in the past there were some respected serpent deities and snakes were occasionally revered as sources of wisdom and protection, affiliated with earth or water as guardians or patrons of the elements; nowadays in our metropolitan, cosmopolitan world, snakes are relegated to pet stores and zoos and squirmy people going "Eww!" at the sight of them. All right, all right, I exaggerate—somewhat. (I mean, come on, look at this particular example of what this general ill-will and disgust and fear of snakes has birthed: Snakes on a Plane.) But the snake has long since lost that glitter of mystique and taken on a much more ordinary place within contemporary society's mindset as a perhaps mildly exotic creature to be looked at and either admired or loathed simply for existing the way it does. The serpent, along the same line, has taken root in modern imagination as a symbol of evil, deceit, poison or venom, death, and/or destruction, and thus a symbol to be feared and despised.
Krispy and I have done a bit of reading on serpents, as perhaps you can tell. There will be serpents in our collaborative novel. More than one. And they will be not necessarily evil, nor even immensely powerful, nor perhaps live up to mythological or contemporary convention, but they will one and all, in some quality or other, be bad-ass.