Stori'r Ddraig Goch :-)

The story of the Red Dragon
The tale of two dragons (one, red and one white), is related to a story that first appears in the 9th Century 'Historia Brittonum'.
The Historia contain a narrative in which the tyrant Vortigern (5th-century British warlord) attempts to build a citadel, but the structure collapses repeatedly. His wise men tell him he must sacrifice a boy born without a father on the spot to alleviate the curse.

Vortigern finally finds such a boy, Emrys (Ambrosius Aurelianus), identified with Merlin in later versions), but Emrys reveals the real reason for the collapsing towers: two dragons, one red and one white, representing the Britons and the Saxons specifically, are buried beneath the foundation.

This story was later adapted by Geoffrey of Monmouth, and thence appears in the Brut y Brenhinedd. Thus, Lludd supplies an origin for the dragons in the Vortigern story.