Dragon lore

15 His [Leviathan's] back is made of rows of shields,
 shut up closely as with a seal. 16 One is so near to another
 that no air can come between them. 17 They are joined one to another;
 they clasp each other and cannot be separated. 18 His sneezings flash forth light,
 and his eyes are like the eyelids of the dawn. 19 Out of his mouth go flaming torches;
 sparks of fire leap forth. 20 Out of his nostrils comes forth smoke,
 as from a boiling pot and burning rushes. 21 His breath kindles coals,
 and a flame comes forth from his mouth (Job 41:15-21).

for from the serpent's root will come forth an adder, and its fruit will be a flying fiery serpent (Isa 14:29).

In that day the Lord with his hard and great and strong sword will punish Leviathan the fleeing serpent, Leviathan the twisting serpent, and he will slay the dragon that is in the sea (Isa 27:1).

Was it not you who cut Rahab in pieces, who pierced the dragon? (Isa 51:9). 

3 And another sign appeared in heaven: behold, a great red dragon, with seven heads and ten horns, and on his heads seven diadems. 4 His tail swept down a third of the stars of heaven and cast them to the earth (Rev 12:3-4).

The origin of the dragon mythos is interesting. At least I find it interesting. It figures in the Bible, but it's culturally diverse. Does it have any basis in reality? 

i) The dragon assumes a stereotypical appearance in occidental and oriental iconography, so there's a danger if modern readers visualize that image when they run across ancient references to "dragons". In early literature, the "dragon" hadn't assumed such a standardized form. It's more composite and varied. When we think of dragons, we think of a huge reptile with a crocodilian or saurian body, clawed paws, spiky dorsal ridge, huge bat wings, head with horns or crest, nostrils that emit smoke and mouth that emits fire.  But ancient descriptions are sometimes more serpentine. 

ii) Because the dragon is a composite animal, more than one animal might underlie the mythical or legendary construct. Two inviting candidates are crocodiles (e.g. Nile, salt-water, black caiman), and constrictors (e.g. reticulating pythons, anacondas). 

The association between dragons and sea-monsters might naturally connect with crocodiles, which are aquatic animals. That link is strengthened by figural references to Egypt or Pharaoh as a dragon, given the notoriety of the Nile crocodile. 

There's also the question of whether venomous snakes play into in the dragon mythos. Although they are physically unimpressive, legendary embellishment might scale them up. 

iii) From what I've read, there's evidence that the habitat of crocodiles used to extend further north. The region was less arid in ancient times. Conversely, Southeast Asia is home to salt-water crocodiles and reticulating pythons. 

iv) What about the Mesoamerican dragon (Quetzalcoatl)? Is that the result of cultural diffusion? Did Indians who settled in South American bring dragon lore with them? Or did it arise independently? For instance, the Amazon river has crocodiles and anacondas. 

v) How do we account for the link between dragons and fire? Dragons function as picturesque metaphors to personify forces of nature, so perhaps free association linked dragons with volcanoes. To take a comparison, consider the legend of fireproof salamanders. 

If now [in the case of] the salamander, which is [only] an offspring of fire.

http://juchre.org/talmud/chagigah/chagigah2.htm#27a

If, therefore, the salamander lives in fire, as naturalists have recorded, and if certain famous mountains of Sicily have been continually on fire from the remotest antiquity until now, and yet remain entire, these are sufficiently convincing examples that everything which burns is not consumed.

http://www.newadvent.org/fathers/120121.htm

If some ancient people thought salamanders live in fire or lava, there'd be nothing incongruous, from their standpoint, about associating dragons with fire. 

Once a particular motif captures the popular imagination, it can develop a life of its own. Consider all the permutations of the vampire that Stoker's novel inspired. 

vi) According to the fossil record, there used to be giant crocodiles (Rhamphosuchus, Deinosuchus) and giant snakes (Titanoboa). On conventional dating schemes, these became extinct before the advent of man. If, however, some of these prehistoric behemoths survived long enough to coexist with humans, that might inspire legendary "dragons". 

vii) If you're a young-earth creationist, you believe dinosaurs coexisted with man. On that view, the dragon mythos might be modeled on pterodactyls.