It remains one of the iconic images of the vampire. Lengthened, piercing teeth, usually (but not always) the bicuspids or eye teeth. Sometimes they stretch to the point of becoming tusks or the saber-teeth of the extinct Smilodon. Other times just a subtle increase in length and reshaping creates the effect. Especially since the Hammer Horror films of the 1950s, 60s and early 70s, our image of a vampire includes fangs. So much so that a major objection many have to the "Twilight" saga continues to be that the Cullens and their undead brethren don't have fangs.
Although, to be fair, neither did Bela Lugosi. One never hears complaints about it, either.
What makes this image just a bit odd is its inaccuracy. Creatures do live on this earth by ingesting blood. Only one of them use fangs to do it. The vampire bat, a native of South America (and hence unknown in Europe for millennia--its inclusion in vampire lore the result of Bram Stoker adding such to his novel "Dracula" albeit getting their size wildly wrong) can easily fit into any child's palm. Interestingly, it does not suck blood. Rather, it pierces the skin of its victim and laps up what spills. As far as fang go, vampire bats have them but not where most fictional undead do. Efficiency dictates these real vampire fangs grow out of the front of the mouth, what in humans we call the front teeth or incisors. Other than "Nosferatu" one is hard-pressed to think of many vampires in films with such a dental design.
Most real vampires don't do anything of the kind. Bed bugs and mosquitoes use a proboscis (actually, the former use a super-fine strand of hair) to pierce their victim's flesh. Ditto fleas. Such a vampire does appear in Brian Lumley's "Wamphyri" novels, the dreaded and disgusting Lord Vasagi the Suck. Something a little similar ended up in the film "Daughter of Darkness" with Mia Sara. Then Guillermo del Toro used that image to disturbing effect in both "Blade 2" and his even-more-skin-crawling (and not-yet-completed) "Strain" trilogy.
This makes for a special horror. As freaked out as we may be about carnivorous mammals out to get us, they at least retain some allure. Wolves and cats, we acknowledge, remain beautiful creatures. Gigantic bugs or even gigantic bug parts don't get that reaction outside of some types of scientists--who almost universally are branded as freakish. But even they tend to shudder at the thought of such things swollen to our scale.
Just as we react with horror at the maw of the common leech. Nothing alluring or positively fascinating about that! Just primal horror at any thought of those things latching onto us and feeding, then having to be cut off and out. (The aforementioned Lord Vasagi underwent an even more horrific metamorphosis, losing his proboscis in favor of becoming in effect one huge mouth--Vasagi the Suck becoming Vasagi the Maw.)
So why do we imagine vampires with fangs? More specifically, why fangs like those of a cat or wolf?
Methinks it might come down to timing, and a change in society which in turn re-designed the building blocks of our dreams. For many centuries Europe had tales of vampires, but in general they fell into three types. A few hinted at ghostly visitations of a sexual nature, the idea being that (male) vampires sought out women, usually wives or girlfriends, to satiate their lust for something other than blood. One of the oldest types centered on the female vampire as a kind of anti-mother, a woman who devoured children usually in a rage directed at men or at a God who took their own children away. Third and most popular (at least in recent centuries) was the hungry, walking corpse who spread disease. In our own horror iconography, this last more closely resembles a zombie.
What changed? The Industrial Revolution, which had among other effects far more people living out their lives in the city rather than in farms. Part of this lay in increased specialization as each field grew more complex and also more productive. But by 1800 something began to be missing from lives, something that had been pervasive until then. Namely, we no longer slaughtered our own meat. More, we didn't see it done. We no longer knew anyone who had seen it done, at least not often. As time went by, this tendency increased. By now our meat rarely even resembles the creature from which it came. Do hamburgers look anything like a cow? Or fish sticks like denizens of the sea? Yet a hundred years past, a pig would likely be served with the head still attached!
Ultimately, this translated into a visceral kind of ignorance. We no longer directly partook of an essential process of life--the killing that allowed our survival, the death out of which our own lives arose. Such a lack, and a feeling that we remained out of touch with that part of our lives, may have helped shape what the vampire became in our imaginations. Rather than a creature of mere lust, rage or hunger, the vampire evolved into an avatar of the process by which we live through death. Everything became heightened. Vampires cannot (or at least do not) have others prepare their meals. More, their sustenance comes not from animals from humans. Life for a time transformed into life eternal, no longer dying themselves save by execution. On top of this grew the internal conflict of the vampire, the feeling of being cursed, of partaking in horror simply in order to survive. Once such tales involved men like Joannes Faust who sold his soul to Mephistopheles, or the title character in "Saint Leon," a gothic novel circa 1800 in which an alchemist finds the secret of eternal life, then regrets it deeply, or the fairy tale "The Soldier Who Cheated Death."
Fangs add to that. The vampire in our current cultural subconscious is Man as Beast, Man as Predator--not a transformation at certain times when he loses his mind a la the werewolf, but every waking moment. Human beings in fact are not a predator species. Biologically we are vastly successful omnivorous scavengers. Pure carnivores eat internal organs, the heart and lungs and pancreas. Our body part of choice tends to be muscles, the sign of a scavenger. Likewise our social structure resembles that of wolves or big cats not at all. One male surrounded by a harem of hard-working females has been an exception in human history, not the rule. And our omnivore nature shows up particularly in our teeth. Not for us the mouth full of grinding molars like a horse, nor the knife-like array common to carnivores. Our teeth show the broad range of our natural diet, plants as well as animals. But the vampire--the vampire really is a predator, a specialized carnivore. Down to our bones we feel the vampire is Not Like Us. Vampires live as predators, and their very nature makes us as their prey.
So, like the predators with whom we are most familiar, and feel we understand most (rightly or wrongly), vampires eventually ended up with fangs. They came to resemble the cat, or the wolf, the tiger or the fox. Beautiful, deadly things. Frightening and fascinating, especially when they smile.