In the autumn the Red Dragon of Wales sleeps in the Wye Valley and every morning it slakes its thirst drinking the cool, clear waters of the Afon Wye. Its hot breath turns the water to steam which turns to smog, or Smaug as we call it around here.
In the Mabinogion story Lludd and Llefelys, the Red Dragon fights with an invading White Dragon. His pained shrieks cause women to miscarry, animals to perish and plants to become barren. Lludd, king of Britain, goes to his wise brother Llefelys in France.
Llefelys tells him to dig a pit in the centre of Britain, fill it with mead, and cover it with cloth. Lludd does this, and the dragons drink the mead and fall asleep. Lludd imprisons them, still wrapped in their cloth, in Dinas Emrys in Snowdonia.
The tale is taken up in the Historia Brittonum. The dragons remain at Dinas Emrys for centuries until King Vortigern tries to build a castle there. Every night the castle walls and foundations are demolished by unseen forces. Vortigern consults his advisers, who tell him to find a boy with no natural father, and sacrifice him. Vortigern finds such a boy (who is later, in some tellings, to become Merlin) who is supposed to be the wisest wizard ever to live.
On hearing that he is to be put to death to end the demolition of the walls, the boy is dismissive of the advice, and tells the king about the two dragons. Vortigern excavates the hill, freeing the dragons. They continue their fight and the Red Dragon finally defeats the White Dragon. The boy tells Vortigern that the White Dragon symbolises the Saxons and that the Red Dragon symbolises the people of Vortigern. If Vortigern is accepted to have lived in the 5th century, then these people are the British whom the Saxons failed to subdue and who became the Welsh.
If you would like to take a journey to Dinas Emrys and visit the Red Dragons lair click here: https://www.nationaltrust.org.uk/craflwyn-and-beddgelert/trails/the-legendary-trail-of-dinas-emrys