A heavier trail rider needs to take special care in picking his or her mount; a shorter horse may be best. Clinician and author Jessica Jahiel, PhD, specializes in teaching beginning riders. She finds that heavier riders often pick the wrong kind of horse.
"A lot of riders look at themselves and say, 'I'm too heavy, I think I need a horse that's 17 hands high, because then I'll look better. And a tall horse can better carry me.' Wrong. Usually, you want a good weight-carrying horse - a nice, sturdy, strong, good-boned horse. A lot of those are going to be in the 15-hand group."
Take a look at Icelandic Horses, Haflingers, and the stock-horse breeds. You'll find bigger-barreled, larger-boned individuals of moderate height that can easily carry your weight. They'll be easier for you to mount, and you'll look just fine.
-Rick Lamb, adapted from Horse Smarts for the Busy Rider
