Unlike the White Walkers, dragons existed in human myths thousand of years before "Game of Thrones." Early on, they were often depicted as serpents, like the dragon that guarded the Golden Fleece from Jason and the Argonauts.
It's not entirely clear where that image of a dragon came from. One theory is that boa constrictors — now an exotic pet — were once a deadly threat to human beings.
"We don't think of them as dangerous today, because we don't sleep in beds of reeds with doors that don't close," Matt Kaplan, author of "The Science of Monsters" and the upcoming "Science of the Magical: From the Holy Grail to Love Potions to Superpowers," told NBC News.
Back when men wandered the wild without machetes or guns, they were a lot more vulnerable to snake attacks. (Even now serpents will occasionally kill people). It's not hard to imagine that these snakes inspired early dragon tales, although the stories might also have been stoked by crocodiles or fossilized dinosaur eggs, or possibly a combination of all of those factors.
But what about the classic Western dragons that breath fire and fly and look more like lizards than snakes? They first appeared in Northern Europe. Between 500 to 600 A.D., a tale emerged about a king named Vortigern who hid in his castle in Wales from angry Saxons. Unfortunately for him, the castle walls kept falling.
In the story, a young boy tells the king that the cause of his falling walls are dragons, who are fighting each other with fire in underground caverns. That boy was Merlin, the famed wizard of Arthurian legend. The reality was more likely connected to rich coal deposits near the castle site in Wales, according to Kaplan.
"If you go digging underground, especially with metal instruments, you are going to start sparks and you're going to start fires," he said.
Later, around 700 to 800 A.D., a fire-breathing creature also appeared in the legend of Beowulf. In Scandinavia at that time, important people were buried with their servants and prized animals. Those bodies would have created a build-up of flammable methane gas.
When grave-robbers opened the tomb, metal tools and torches in-hand, they could have sparked the flames that inspired dragons in Scandinavian poetry. That would have also reinforced the idea that dragons guarded treasure.
A professor named J.R.R. Tolkien fell in love with the tale of Beowulf, wrote his own translation, and then created the flying dragon Smaug for his 1937 novel "The Hobbit."
"It's a classic Western vision of a dragon," Kaplan said. "Tolkien really solidified the idea of what a dragon should be, and 'Game of Thrones' uses that Tolkien-esque vision of a dragon."
So there you go, "Game of Thrones" fans—the mighty Balerion was inspired by by Scandinavian grave-robbers who were scared by exploding methane gas.