Actually, there's no real evidence (other than what you can find repeated online) about the existence of a red dragon god called Dewi in Wales. It doesn't even have any connecection to the Welsh word for dragon other than the initial letter. The red dragon comes from Roman standards that were flown in occupied Wales, and from a story linking a red dragon to Wales, a white one to the English, and the battle between them symbolizing the battle between the two. The red one won. =D.
See also: https://en.wikipedia.org/wiki/Trethevy