I was specifically referring to horse colic, which among the many many many causes are things like refusing to exercise after eating (because it is cold out and they don’t wanna), eating at a time they don’t normally eat, and the ever popular eating something they do not usually eat. Colic, even things like gas colic, can lead to death shockingly quickly. This is part of the reason that a break to the horse’s leg at or below the cannon is fatal: because exercise is critical to a horse’s digestion, the lack of any muscles to immobilize or support an injury means that a breakage will never be able to heal and a horse is faced with dying from colic or torturous pain as they die from an infection and internal injuries resulting from walking on a broken limb. It’s a horrible way to die.

Riding a horse is a complex topic and there are studies for days about it’s impact to a horse’s overall health. I don’t really wanna get into that because it’s extremely complex, there is no one exact right answer and I’m generally of the opinion that there’s absolutely no point to horses (let alone riding one of the damn things) so why spend more time than needed thinking about the fuckers… However I will point out that I think you’re confusing horse anatomy with elephant anatomy - unless you’re riding bareback, the force of a rider is not carried on the horse’s spine (nor is the force of a harness) it is carried on their shoulders, and due to the shape of horse gaits their spines are much more able to support vertical loads than many other herbivores (horses just moving put absolutely insane dynamic loads on their bodies, the addition of a static load in the form of a rider is extremely small in comparison. This doesn’t address jumping which… is it’s own complex topic and I just flat don’t think it’s okay to do high jumps on a horse).

(The most common injury to a horse’s back, Baastrup’s Disease (“Kissing Spines”), doesn’t have a known definite cause but ill-fitting tack is known to exacerbate it (by shifting the weight off the shoulders and onto the mid/lower spine). An unridden horse can develop it, and it affects wild horses just as severely as domestic ones (though I cannot find a source on the rate, I suspect because studying the spines of a wild horse is a pretty tricky operation))

In general, exercise in horses is the same thing as in humans - we can carry weights, even uncomfortably large weights, and as long as we’re not doing that all the time our bodies will not suffer unduely. Some of us flat shouldn’t do that, some of us absolutely live for that (hikers, runners, gym nerds, Belgians) and when we do do it, the most important aspects are to make sure we do it with good ergonomics, don’t exercise over an injury and don’t spend all of our time doing it.

And also I hate horses.