Like the flag of Bigouden Country it was designed by Bernard Le Brun, and, according to the accompanying information sheets, approved by the Breton Vexillological Society on Sunday 10 November 1996; confirmed on Sunday 16 March 1997 and officially hoisted for the first time on 29 March 1998 at Trézelan.
As I understand it, the dragon mentioned in the text is technically a wyvern - a heraldic beast which I have seen described as having "the front half of a dragon and the rear half of a serpent"; or alternatively as "a winged two-legged dragon with a barbed tail".
"The red dragon is the emblem of saint Tudwal, founder and patron-saint of Trégor. It can also represent King Arthur who, according to legend, was born in Trygger (in Breton Treger) in British Cornwall and whose tomb is shown in the iconography and the legends with such an emblem. The base of the flag is a simplified version of the flag of saint Yves, well-known in the area. A banner with the dragon accompanying king Arthur can be seen on a bas-relief in the romanesque church of Perros-Guirec (Perroz-Gireg).