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, we can at least infer that he was probably a Cornish knight and running around wild in the forest in that part of the country.