*grins* I figure the primary reason you don't see horses is because they're bloody hard to draw and not something most comic book artists practice at. Personally, I don't think Xavier would waste money on keeping horses, not with his greater goal of policing the entire world's mutant population. It would be something ill afforded given other priorities.

However, with that being said... having horses around is damn cool and doesn't detract from anything so why not? :)