X-Men is a part of the Marvel Universe and has been since the beginning. It always drew stuff from outside the X-Men books. The problem is that the stuff that got drawn in ended up becoming better known as "X-Men stuff" afterward. Adamantium was created for an Avengers story involving Ultron. Mystique and Deathbird were originally Ms. Marvel (Carol Danvers) villains. Also, Rogue's better known power-set came from Carol herself. Sabretooth was an Iron Fist villain. Wolverine first appeared in an issue of Incredible Hulk. Arcade was created for a Marvel Team-Up story that teamed Spider-Man with Captain Britain. Not to mention the whole Captain Britain mythos itself. Ka-Zar was a character way back in the Golden Age. Longshot and Mojo come from the Longshot miniseries which guest-starred Doctor Strange (a guy who frequently invokes the "Crimson Bands of Cytorrak", which are connected to the Juggernaut). The thing is that the X-Men franchise just kind of swallows this stuff up. Admittedly, part of the reason it's so easily swallowed is because most of it is more on the obscure side. Claremont in particular liked folding stuff from the less popular books he worked on in the '70s and '80s (Iron Fist, Ms. Marvel, Captain Britain) into X-Men where he could revisit it.