It might be much easier to load a 200-pound muley buck onto the back of a horse if the pack animal is gotten as low to the ground as possible.
It's also a lot easier to give the horse joint arthritis, muscle sprains and general soft tissue injuries if you ask it to do something like that. Horses aren't designed to weightlift loads off the ground, even if you can "make" them do it.
Just like in human beings, putting the skeleton under strain with the joints bent to that extent is asking for busted cartilage and arthritis. Unlike camels, horses are also prone to quick movements if they take a fright, and this could lead to serious injury under load in that position.
Unless you are an Olympic bodybuilder, you can't just sling that big deer carcass over the back of your pack animal while at a full standing position.
But you expect the horse to be an Olympic bodybuilder on your behalf???
You might be better off with a 4-wheeler.
If the horse could be made to get down low, I would think one could simply drag the carcass over the horse's back then secure the load properly. It reminds me of cowboy films where the bounty hunter throws the dead bodies of wanted outlaws over the backs of horse in his string then parades them into town.
Yes, let's all take our cues on the humane treatment of horses from what we see on cowboy films.
That wording of yours I've highlighted above, and the idea that you can "command" a horse, deserves a reminder that horses are sentient beings and don't owe you anything - and that if we're going to work with horses, we owe them respect.