When were the X-Men first introduced by Marvel?
List of X-Men comics
The X-Men are a team of superheroes appearing in American comic books published by the Marvel Comics. The X-Men first appeared in the self-titled X-Men comic, cover dated September 1963. Due to the X-Men's immense popularity, Marvel has launched dozens of spin-off series, called "X-Books," throughout the years.
Ultimate Marvel
Ultimate Marvel, later known as Ultimate Comics, was an imprint of comic books published by Marvel Comics, featuring re-imagined and updated versions of the company's superhero characters from the Ultimate Universe. Those characters include Spider-Man, the X-Men, the Avengers and the Fantastic Four. The imprint was launched in 2000 with the publication of the series "Ultimate Spider-Man" and "Ultimate X-Men", providing new origin stories for the characters.