Why Dragons Fear Lighting
An excerpt from Byzantine imperial court intellectual Michael Psellos (1018-ca.1075):
...Is it true what what some people say, that the race of dragons fears lighting-fire, or even that this fire is actually waging a war against dragons and that it targets them with an irresistible momentum, propelled by almost voluntary powers? And what are we to make of the sights we observe in the blaze of lightning, which sometimes imitate one animal, and sometimes another?....
As for dragons, I have not yet, to this day, come across any treatise that explains, in the terms of natural science, why they fear lightning. So once the first person has done so [that is, Psellos himself] —and I too will refer it to my conception of nature —then perhaps another person will give a more exact interpretation of it.
I, then, say that the race of dragons is dry by nature and fiery in substance. Even their eyes blaze forth with fire and the venom they vomit forth is smoky and it dissolves any bodies that it touches. Thus, dragons are vulnerable to fire on account of their dryness and they are liable to be burned up. Hence, they fear lightning and leap through the air looking for enclosed spaces under the earth, especially around lakes and cisterns, since, as I said, they are vulnerable to fire on account of their dryness and can be burned up by lightning fire, even from a distance. That is why, wherever a dragon appears, hissing as it passes through, lightning strikes there, ravaging the adjacent areas. It is often the case that rattling, hissing, and grunting sounds are heard even without a dragon present...But if a malicious demon should take on the body of a dragon and, pursued by fire, should lead the beast to those places where the demon was already accustomed to spread its malice, this account is not to be rejected entirely out of hand. For this is the ancient agenda of that race, on account of which it formerly drove us out of Paradise and is now attempting, again through its ill-omened malice, to remove us from the earth as well.
(The Penguin Book of Dragons, pgs. 120-121)
The details about dragons being afraid of lightning are quite strange, but I think it is more important to take notice of how Psellos describes, in the highlighted section, dragons can be possessed by demons. It confirms the opinions of some who say that dragons spoken of in historical accounts were, at least in part, of demonic origin.