The Welsh flag is a a white field with a large red dragon standing on a patch of green grass. The red dragon (Y Ddraig Goch) is the most prominent Welsh symbol, and has been associated with Wales for centuries.

Until the 4th c. Britain was occupied by Roman cohorts; which emblem was the draco. When the Roman Empire broke up, the dragon naturally became the standard of many Briton and Saxon landlords. A golden dragon was the principal war standard of the Saxons of Wessex, and was carried by them at the battle of Burford in 752.

The dragon standard was adopted by the Normans after the battle of Hastings in 1066, battle that ended in the defeat of Harold II of England by William, duke of Normandy, and established the Normans as the rulers of England..
A dragon standard was taken on the Third Crusade by Richard I in 1191.
 

 
Cadwaladr (7th c.), heroe of Wales, used to raise a dragon and it has been accepted as a distinguishing feature when Welsh archers, dressed in white and green, served in the English army at the battle of Crecy (1346).
King Henry VII (Henry Tudor), wanted the dragon to be put on his banner and to become the symbol of Wales, along with the colours of the Tudor family: white and green (Until this time it was probably golden). He flew the red dragon, when he marched through Wales on his way to Bosworth battle in 1485.
 
The Welsh dragon was used in the royal arms in the 15th Century, but with the Union of the Crowns of Scotland and England in 1605, when James VI (Scotland) became James I (England), the Welsh influence seems to have disappeared. This was perhaps because by this time Wales was simply considered as part of England.

Nowadays, the Red Dragon sitting squarely on the green and white background of the Tudors, compose the Welsh flag, since Queen Elisabeth II acknowledged it in 1959.
 

 
The origin of this symbol is found in early Welsh legends and history, such as the “Historia Brittonum”

Centuries ago in the time after the Romans had left Britain, the King of all Britain was called Vortigern (or Gwrtheyrn in Welsh). He was under attack from the Saxons, so he decided to build a fortress, but every time the fort walls were built they fell down.
The wise men were consulted, who said that it was the work of a bad spirit, and that a child without a father should be sacrificed and its blood poured on the foundations to keep the spirit happy. The castle could then be built in peace. And so men were sent in all directions to seek such a child.
 
This boy was Myrddin Emrys- Merlin. When he was about to be sacrificed, he said that two dragons (one white and one red) lived in a cave under the hill, and it was their fighting that was causing the walls to fall down.
 
Merlin was proved right and the dragons were found. The dragon of the Britons is identified as red, while the white dragon represents the Saxons. Merlin prophecies that the white dragon at first prevailed but in the end, the red dragon overcame the white and would be the final victor. (The Welsh would ultimately, overcome the foreign invaders, after a long period of adversity). This is believed to represent the conflict in the 5th and 6th centuries between the British Celts (who later became the Welsh) and the invading Saxons.
 
After Vortigern’s death, Merlin became councelor of three great kings: Aurelio, Uter Pendragon, and the most famous of all, King Arthur, son of Uther Pendragon.