Those Horses Are Actually Mules
It's a small point, but the animals that carry the overweight tourists in Zion National Park are mules. How else could they make the trip? They do not weigh wannabe riders in Zion, but they do so on the North Rim. Not to worry. The animals are quite powerful and do a wonderful job of navigating down the incredible steepness of the Grand Canyon. I recommend taking a mule ride at one (or both) venues if you are not over their weight limit of around 200 pounds -- and you DO need to make a reservation to be guaranteed a ride.