Mendez XII was a mutant leader from the Mendez Dynasty. Nothing is known about this mutant, except for the fact that a bust of him was constructed and placed upon a plinth within an air tunnel passageway outside of the mutant cathedral. 
Possibly, Mendez XII was a biological descendant of the first Mendez who occupied the West Coast Forbidden Zone in the late 20th/early 21st century. It can also be assumed that Mendez' genealogical line extended into the 40th century, culminating with the Dynasty's final inheritor, Mendez XXVI. Whether or not Mendez was the mutant's true birth name, or merely an honorarium is unknown.