Don Juan Matus is a major character in the series of books by Carlos Castaneda. Assuming he was a real person (as claimed by Castaneda), his real name was changed to maintain his anonymity.
In Castaneda's books, Don Juan Matus was a Yaqui Indian whom he met during anthropological field work around the US-Mexico border. On subsequent visits, Don Juan revealed himself to Castaneda as being a sort of shaman who had inherited (through a lineage of teachers) an ancient Central American practice for refining one's awareness of the universe.
Taisha Abelar and Florinda Donner-Grau also wrote about the same Don Juan Matus, although he went by different pseudonyms in their books such as Mariano Aureliano.
In all of these books, Don Juan Matus was a nagual who was leader of a group of practicioners of tradition of perceptual enhancement.
See Carlos Castaneda for a list of books in which Don Juan Matus appears.