The draft horses that we're talking about in question here are tall animals with a high centre of gravity. Over as long a flight as we're talking about, they can become fatigued during that travel and would be inclined to rest in one of two ways. One would be standing on three legs, a tripod, which can be hard to do while maintaining balance in a moving airplane. The other would be lying down, which we may not want them to do. However, that would require the horse's inclination to lie down. Of course, horses prefer to not lie down in an unfamiliar environment, with noisy stimulation. We end up with two options, neither of which is great, and that leads to horses that don't properly rest on a plane in transport. Consequently we end up with horses that are exhausted, potentially injured and potentially lame at the end of that travel.
On March 21st, 2024. See this statement in context.