Rex originally appeared in the Age of Apocalypse and served as En Sabah Nur's majordomo. His 616 variant appeared in X-Man after Nate Grey crossed over into the mainstream universe. He was the Sugar Man's right hand man and had a short but impactful run.
616 Reading List:
X-Man Vol 1 No. 6
X-Man Vol 1 No. 7
X-Man Vol 1 No. 8
Age of Apocalypse Reading List:
Astonishing X-Men Vol 1 No. 1
Astonishing X-Men Vol 1 No. 2
Amazing X-Men Vol 1 No. 2
Factor X Vol 1 No. 3
Astonishing X-Men Vol 1 No. 4
Weapon X Vol 1 No. 4
X-Men Omega Vol 1 No. 1