well the answer is on this website here's a clip but follow the link as well

http://www.britannia.com/celtic/wales/facts/facts2.html

The red dragon of Wales (Y Ddraig goch) goes back a long time, long before the Union Jack was ever put together. As a national symbol for Wales, it predates its adaptation by the Tudors. The dragon is perhaps the very first mythical beast in British heraldry. Legend has Macsen Wledig and his Romano-British soldiers carrying the red dragon (Draco) to Rome on their banners in the fourth century. It was adopted in the early fifth century by the Welsh kings of Aberffraw to symbolize their authority after the Roman withdrawal. By the seventh century, it was known as the Red Dragon of Cadwallader, forever after to be associated with the people of Wales. The ninth century historian Nennius mentions the red dragon in his Historia Brittonum and it was referred to by Geoffrey of Monmouth in his Historia Regum Britanniae written between 1120 and 1129.