Cait Stevenson on Medievalists.net:

Seven Things You Didn’t Know About Medieval Dragons

1. Medieval people understood what a “dragon” was.

I say dragon, and you visualize a giant fire breathing, flying lizard with legs, maybe claws, maybe a voice. I know, and you know. Dragons are that iconic a cultural image. The same was true of the Middle Ages. As Paul Acker showed, Norse sagas spend basically no time describing the physical characteristics of their dragon foes—despite the physical form of the dragon being crucial to the hero’s contest against it and the audience’s ability to follow the battle.

2. Or rather, medieval people understood what dragons, plural, were. 

In the thirteenth-century Norse poem Fáfnismál, Sigurd digs himself a pit where he knows the greedy dwarf-turned-dragon Fáfnir will slither. Our hero stabs upwards into the dragon’s belly. This feat makes no sense to an audience expecting a monster who lumbers on legs or takes to the skies. Just like we “know” what a dragon is from general cultural awareness, medieval people knew.

Fáfnir is, for all intents and purposes, a giant snake. He spews clouds and rivers of venom like a giant snake. A frequent attribute of dragons in medieval bestiaries (descriptions of animals and their characteristics meant as moral lessons for people) recounts their ability to kill animals as large as elephants via constriction and suffocation… you know, like a giant snake. But nobody blinks when Chretien de Troyes’ great dragon in Yvain is “so full of evil that fire leapt from its mouth,” or when the Norse adaptation of Chretien’s Arthuriana lets its dragon dispense with the cooking and get right to the eating. Sometimes dragons are specified as flugdrekar – flying dragons – and sometimes simple drekar and ormar (“worms”) are described as taking to flight. What medieval people understood by “dragon” was a much wider range than the color-coding schemes of modern fantasy. Which brings us to the most excellent variety of medieval dragon:

3. Before they acquired the ability to fly, medieval dragons dropped out of trees onto people’s heads.

As Pliny the Elder’s Natural History, an important source for medieval encyclopedic and bestiary writing, has it:

The iaculus throws itself from the branches of trees; dragons are dangerous not only to the feet but also fly like a missile from a catapult.

Isidore of Seville in the early seventh century clarified that the iaculusindeed takes its name from the javelin, and the Norse Rómverja sagadraws out this point in excruciating detail:

struck under the cheek that man who was called Paulus and the serpent flew straight into his head and out of his cheek

Read the rest at the link.