There are three main places in Cozumel that we know of. Carnival books with Rancho Buena Vista and I can't book privately with them from what I can tell. There is also Ranchos Palmitas and Aventuras Naturales.
Each place offers the same basic excursion for the same price, but I am having a hard time finding reviews for the places, including Carnival's. My husband and I are looking for a 2-3 hour ride where we can trot or canter for some part as we both very much enjoy horseback riding.