For most of history, the Welsh dragon was not a very common symbol. In fact, it was flown by only one king, Cadwaladr ap Cadwallon, who reigned from 655-682 AD. It was so distinct that his flag came to be known as “the Red Dragon of Cadwaladr”.  Today, it is known as the ‘Welsh dragon’ and the the Welsh flag looks like this:

 

(my The Last Pendragon Saga is about a mythic version of Cadwaladr)

Within Welsh mythology, the story of the two dragons, one red (for the Welsh) and one white (for the Saxons) fighting beneath Dinas Emrys dates back to Geoffrey of Monmouth, writing in the 12th century.

The coat of arms of the Welsh princes in the 13th century was actually this:

With the death of Llywelyn ap Gruffydd and the complete suppression of everything that had belonged to or symbolized the Princes of Wales, this banner disappeared from royal heraldry.

It wasn’t until Henry Tudur (Henry Tudor/Henry VII) of England, in a quest for legitimacy to both the Welsh and the English, took the Red Dragon and made it his own that the dragon and Wales became synonymous. In flying the flag, he claimed that he was a direct descendent of Cadwaladr, but he laid the dragon over the green and white colors of the House of Tudor.  Henry then marched through Wales on his way to seize the crown on 22 August 1485 when he defeated Richard III at the Battle of Bosworth Field.  The flying of the flag was very deliberate.  For the Welsh, it was Cadwaladr, not King Arthur, who was to ‘return’, to save the Welsh from their enemies. Henry Tudur very deliberately took up that mantle.

In an tragic twist, Henry’s first son was named ‘Arthur’, but he died in 1502, and was succeeded by his brother, Henry (becoming Henry VIII) as the heir to the throne.