[Phil Masters]

I recall a very tongue-in-cheek theory that all the horns and frills on a dragon's head are non-essential horny growths, and the dragon's fabled "hypnotic gaze" is largely a set of instinctive movements and postures that persuade opponents to follow the (usually sensible on most creatures) tactic of striking for the head. With a small reptilian brain buried deep in a thick skull, this suits the dragon pretty well. While the dragon hunters are busy hacking off those decorative frills (which regrow well enough), the dragon can chomp on them at leisure.

Then the dragon hunters who survive anyway go home, retire, and tell the youngsters to be careful - "Dragons have one invulnerable spot!". But the kids, being kids, think the old fool must've got that the wrong way round, and dragons must have just one vulnerable spot.