X
- X-51, Machine Man in 2001: A Space Odyssey #8, July 1977
- X-23 in Nyx #3, February 2004
- X-Man (Nate Grey) in X-Men: Alpha
- The X-Men in X-Men #1
- Xemnu the Titan in Journey into Mystery #62
Read more about this topic: List Of First Appearances In Marvel Comics Publications