Duke's horse is described as a roan several times. It's not a roan but appears to be a chestnut or sorrel. A roan horse has numerous white hairs mixed in with its main color all over its body, which gives it a much lighter look.
When rancher Pete Ferguson is slain, one of his ranch hands is seen fleeing. In the full body shot he is quite portly, but in the shot of him mounting his horse is filmed from the waist down in a (failed) attempt to hide the fact that it is a stuntman mounting the horse in his place. The stuntman is noticeably thinner/more fit and taller.