The X-Men would never have fit in Phase 1 of the Marvel Cinematic Universe, and it’s a lot better for them to be introduced in the MCU's Phase 4. Ever since Patrick Stewart’s Professor X appearance in the Doctor Strange in the Multiverse of Madness trailer, speculation has been rife regarding exactly how the X-Men can finally materialize in the MCU. However, an alternative post-credits scene from Iron Man suggests the X-Men were supposed to be part of the MCU from the very beginning — potentially being part of the original Avengers team. Yet it has only been in Phase 4 that mentions of mutants have officially appeared, such as Evan Peter’s Quicksilver appearance in WandaVision, although this was said to be part of Wanda’s imagination and not the Quicksilver in X-Men.
The X-Men were originally owned by 21st Century Fox, so Disney, and Marvel, did not have the rights to the X-Men until they acquired 21st Century Fox in 2019. Fox built their own world for the X-Men, which led to spin-off films such as The New Mutants. Throughout the X-Men universe, Charles Xavier's team often fought to protect themselves as mutants and stay hidden until they could live peacefully alongside humans in a future that never materialized for the X-Men.
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Phase 4 is massively expanding the MCU, and the introduction of the Multiverse gives far more possibilities for introducing the X-Men as a result. The MCU's Phase 1 was primarily Earth-based, with the only appearance of alien species in the MCU being Asgardians (who humans knew from old legends and stories) and the Chitauri attack in The Avengers. Although the mutants could have still been introduced in Phase 1, it is unlikely that
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