"All the white dragons originally come from the moon, as you probably know; but this one had been to the world and back, so he had learned a thing or two. He fought the Red Dragon in Caerdragon in Merlin's time, as you will find in all the more up-to-date history books; after which the other dragon was Very Red. Later he did lots more damage in the Three Islands, and went to live on the top of Snowdon for a time. People did not bother to climb up while that lasted - except for one man, and the dragon caught him drinking out of a bottle. That man finished in such a hurry that he left the bottle on the top, and his example has been followed by many people since. A long time since, and not until the dragon had flown off to Gwynfa, some time after King Arthur's disappearance, at a time when dragons' tails were esteemed a great delicacy by the Saxon Kings."
(Roverandom)