There are a number of places for horseback riding here on the island, however, I'm not sure how you can contact them in advance.
I do remember that Sherri at cozumelinsider features a horseback riding tour along with a several other "out of the water" tours.
Not sure about the price, but you can go to cozumelinsider.com and check it out. Generally horseback riding lasts about 3 hours, so you'll have lots of time to do other stuff.
Ross in Cozumel