[Teleri]

Not to mention the fact that a horse from Rohan might be bigger than your average horse.
Though you could compare Merry's hieght to a jocky, we must remember that this is War. Perhaps Merry could manage a horse without armour, but throw in that, plus pikes, shields, and swords, and the situation becomes a lot more complicated. He would need a sword much too big for him to even be able to deal a blow.