I think it was Tree beard that said at one point that all things can speak, but with there own tongs (Aka, the talking Trees, Old man willow, the badgers in "the adventures of Tom Bombadill ect). It is likely that the Horses had there own tong, therefore it is not beyond the realms of possibility that one of the Mearas at least could, with time, understand or comprehend the tong of men.
As has been said, the Mearas would only allow kings of the mark to ride them.
Perhaps Sahdowfax still had some pride about the tradition his kin had, and that is why he made Gandalf ride him bare back. Or did all have to ride the Mearas bare back.... I'll have to check! (Digs through large collection of books)
