What is "teulyddog"

Saint Teulyddog (; and Thelaucus) was a medieval Welsh saint.

Accounted a disciple of Saint Dubricius, Teulyddog is said to have fled with many others to Brittany during the Yellow Plague of Rhos (y Fad Felen) in the 540s. Returning to Wales, he associated himself with Saint Teilo and was credited with the establishment of the clas (ecclesiastical settlement) in Carmarthen, which took its medieval name, Llan Teulyddog, in his honor. Following the Norman invasion of Wales, his patronage of the town was assumed by John the Evangelist.