When Abel is getting Mark posed the second time, there’s a horse standing by the stairs next to the hotel, way in the background. But watch in the close up and wide shots of Mark putting on his hat right after Abel took the photo. The horse isn’t as near the stairs. It is still there, but facing the hotel in both shots. In Mark’s close up, the four hooves are under the horse on our right’s belly. But in the wide shot, it’s moved again and is almost completely blocked by the horse with its head next to Col. Whiteside's right arm. Look closely though and you’ll see its rump. (It’s very visible on a TV screen if you can’t see it here.)
Thanks Ann Marie!