The dragon 'motif' is not meant to be reminiscent of a mythical dragon encounter, like with St George. It's from the 'Draco', more like the ornamental Chinese dragons you see at festivals. The Romans would go into battle with a metal draco with glowing coals in its mouth, at the head of the army. Hence, the breathing fire. It spans the first second and third millennia as a Welsh symbol; though obviously not as the flag, which is quite modern, I believe.