Namor, the first super-hero in all of Marvel (he debuted in the pages of Motion Picture Funnies Weekly in 1939), has a sketchy history with his mutant background. As far back as the early days of The X-Men in the ‘60s, both Professor X and Magneto postulated that
Namor was one of their kind. At other times the idea of him being a mutant was downplayed, and his powers were attributed to being the child of an Atlantean and a human. This failed, however, to explain away the tiny little wings on his ankles and his ability to fly (Atlanteans, like Namor, can breathe underwater… but they can’t fly).
Following the events of Secret Invasion, Norman Osborn (formerly the villainous Green Goblin) is given immense governmental power. He creates “Dark” versions of several super-hero teams to act as his enforcers, besmirch the names of the regular members of the teams, and to act as heroes that he is able to control. Among the Dark X-Men is Namor, who in comics again had his mutant genetics played up. Namor eventually betrays Osborn and sides with Cyclops as a legitimate member of the X-Men, just as Cyclops is creating an island refuge for mutants off the coast of San Francisco called Utopia. Following the lead-up to and events of the most recent Secret Wars, Namor no longer appears to be an X-Man.