Not at all. I traveled from Olivine to Cianwood on a Bouffalant, and from Ecruteak to Mahogany on a Snorlax. Even my Furret could theoritically do the job, if I could find a way to sit on her back comfortably.

[And aren't those all lovely mental images? Just add some green to the Bouffalant's afro, a few crushed berries to the Snorlax' chin...]