Two entries with the name Mathan.
Mathan
Mathamas, Mathans
One of the many Saxon kings to invade northern Britain at the beginning of Arthur’s reign.
Under King Hargadabran he fought Arthur at Clarence. Arthur’s Sir Dodinel killed him.
Source
Le Livre d’Artus | Early 13th century
Mathan
Marthan, Marthaut, Matan, Mathamas
A knight who fought before Clarence on the rebel kings’ side.
He is probably identical with the Saxon king Mathan in the entry above.