The Legend of Merlin and the White Dragon
The terms dragon, drake, serpent, worm, were more or less interchangeable in northern Europe, where even now you may hear described to you a fabulous wurm-bett, or serpent's bed, as the place of gold with a dragon-guardian. So it was in Britain, where this creature was associated with the exploits of the Round Table; for we find the following among the Arthurian legends which are more particularly Welsh.
Merlin, the magician, was asked by King Vortigern (fifth century), how to render stable a tower of his castle which thrice had tumbled down. Merlin explained that the trouble lay in the fact that the tower had been built over the den of two immense dragons, whose combats shook the foundations above them. "The king ordered his workmen to dig," as Bulfinch tells it, "and when they had done so they discovered two enormous serpents, the one white as milk, the other red as fire. The multitude looked on with amazement till the serpents, slowly rising from their den, and expanding their enormous folds, began the combat, when every one fled in terror except Merlin, who stood by, clapping his hands and cheering on the conflict. The red dragon was slain, and the white one, gliding through a cleft in the rock, disappeared."
This incident is reputed to have taken place on an isolated rocky eminence in Carnarvonshire, where remains of extensive prehistoric stone-works are still to be seen, says Rhys; in truth it is, of course, purely mythical.
From DRAGONS AND DRAGON LORE by Ernest Ingersoll (1928)