An old Welsh god.
The official emblem of Wales, a red dragon, is derived from the Great Red Serpent that once represented the god Dewi.

He was Christianized as St. David.
As Saint David...Little is known of his life. According to the hagiography (c. 1090) by the Welsh scholar Rhygyfarch, he was the son of the chieftain Sant, who raped David's mother, St. Non. Educated at Henfynyw, Cardigan, he seemingly took a prominent part in the synod of Llanddewi-Brefi (in Cardigan) to suppress the heresy of Pelagius (q.v.) and presided at the Synod of Victory held later at Caerleon-on-Usk, Monmouthshire, which supposedly defeated the Pelagian heresy in Britain. More certainly, he moved the seat of ecclesiastical government from Caerleon to Mynyw, which still, as St. David's (Ty-Dewi), is the cathedral city of the western see. David founded numerous churches throughout South Wales (more than 50 named for him existed in the 20th century). His shrine at St. David's became a notable place of pilgrimage. His canonization by Pope Calixtus II (c. 1120) is unproven. Source: Saint David information from Encyclopedia Britannica

Tracking down information about the origional Welsh Dewi still in progress. If you have any please feel free to share it, if not patience is not a virtue, feel free to help me track this elusive God down.