Matto Le Bruene
A giant, noble Cornish knight. Gaheris, Gawain’s brother, defeated him over a lady, driving Mathan insane. He roamed wild through the forest of Morrois. When Tristan similarly went insane, he was at first mistaken for Mathan.
Ah, said King Mark, that is Sir Matto le Breune, that fell out of his wit because he lost his lady; for when Sir Gaheris smote down Sir Matto and won his lady of him, never since was he in his mind, and that was pity, for he was a good knight.
As it happens, King Mark has here mistaken Dagonet’s account of a crazed Tristram for Sir Matto. From this tantalizing reference, which seems to be Malory’s only mention of Sir Matto.
Sources
Prose Tristan | 1230-1240
Le Morte Darthur | Sir Thomas Malory, 1469-1470