List of legendary rulers of Cornwall Wikipedia
"Duke of Cornwall" appears as a title in pseudo-historical authors as Nennius and Geoffrey of Monmouth. The list is patchy and not every succession was unbroken. Indeed, Geoffrey repeatedly introduces Dukes of Cornwall only to promote them to the Kingship of the Britons and thus put an end to their line as (merely) dukes. As adjuncts or supporting roles to the kings of the Britons, the legendary dukes of Cornwall are considered part of the vast Matter of Britain.
The list is more often thought of as a conglomeration of various Celtic rulers, Celtic warlords, and mythical heroes. If the lists of kings of Britain are legendary, then the list of dukes must be considered still more a genealogical and historical legend with no solid basis in the view of most historians. (Even within Geoffrey's History, the title of these rulers fluctuates between "duke" (dux Cornubiae) and "king" (rex Cornubiae).)