Redditor /u/coffeeincluded:

Horses. Dear god, horses.

First off, horses are obligate nasal breathers. If our noses are stuffed up we can breathe through our mouths. If our pets' noses are stuffed up (except for rabbits, who are also really fragile but unlike horses aren't stuck having only one baby a year) they can breathe through their mouths. If a horse can't breathe through its nose, it will suffocate and die.

Horse eyes are exquisitely sensitive to steroids. Most animal eyes are, except for cows because cows are tanks, but horses are extremely sensitive. Corneal ulcers won't heal. They'll probably get worse. They might rupture and cause eyeball fluid to leak out.

If you overexert a horse they can get exertional rhabodmyolysis. Basically you overwork their muscles and they break down and die and release their contents. Super painful, and then you get scarifying and necrosis. But that's not the problem. See, when muscles die hey release myoglobin, which goes into the blood and is filtered by the kidneys. If you dump a bucket of myoglobin into the blood then it shreds the kidneys, causing acute renal failure. This kills the horse. People and other animals can get that too but in school we only talked about it in context of the horse.

Horses can only have one foal at a time. Their uterus simply can't support two foals. If a pregnant horse has twins you have to abort one or they'll both die and possibly kill the mother with them. A lot of this has to do with the way horse placentas work. EDIT: There are very, very rare instances where a mare can successfully have twins, but it's sort of like the odds of being able to walk again after a paralyzing spinal injury.

If a horse rears up on its hind legs it can fall over, hit the back of its head, and get a traumatic brain injury.

Now to their digestive system. Oh boy. First of all, they can't vomit. There's an incredibly tight sphincter in between the stomach and esophagus that simply won't open up. If a horse is vomiting it's literally about to die. In many cases their stomach will rupture before they vomit. When treating colic you need to reflux the horse, which means shoving a tube into their stomach and pumping out any material to decompress the stomach and proximal GI tract. Their small intestines are 70+ feet long (which is expected for a big herbivore) and can get strangulated, which is fatal without surgery.

Let's go to the large intestine. Horses are hindgut fermenters, not ruminants. I'll spare you the diagram and extended anatomy lesson but here's what you need to know: Their cecum is large enough to shove a person into, and the path of digesta doubles back on itself. The large intestine is very long, has segments of various diameters, multiple flexures, and doubles back on itself several times. It's not anchored to the body wall with mesentery like it is in many other animals. The spleen can get trapped. Parts of the colon can get filled with gas or digested food and/or get displaced. Parts of the large intestine can twist on themselves, causing torsions or volvulus. These conditions can range from mildly painful to excruciating. Many require surgery or intense medical therapy for the horse to have any chance of surviving. Any part of the large intestine can fail at any time and potentially kill the horse. A change in feed can cause colic. Giving birth can cause I believe a large colon volvulus I don't know at the moment I'm going into small animal medicine. Infections can cause colic. Lots of things can cause colic and you better hope it's an impaction and not enteritis or a volvulus.

And now the legs. Before we start with bones and hooves let's talk about the skin. The skin on horse legs, particularly their lower legs, is under a lot of tension and has basically no subcutaneous tissue. If a horse lacerated its legs and has a dangling flap of skin that's a fucking nightmare. That skin is incredibly difficult to successfully suture back together because it's under so much tension. There's basically no subcutaneous tissue underneath. You need to use releasing incisions and all sorts of undermining techniques to even get the skin loose enough to close without tearing itself apart afterwards. Also horses like to get this thing called proud flesh where scar tissue just builds up into this giant ugly mass that restricts movement. If a horse severely lacerated a leg it will take months to heal and the prognosis is not great.

Let's look at the bones. You know how if a horse breaks a leg you usually have to euthanize it? There's a reason for that. Some fractures can be repaired but others can't. A horse weighs thousands of pounds and is literally carrying all that weight on the middle toes of their legs. They are simply incapable of bearing weight on three legs. And a lot of that is because of...

Laminitis. This killed Barbaro and Secretariat. Barbaro would have made it through the broken leg but he got laminitis in his other legs. First, a quick anatomy lesson. The horse hoof is like our fingernails, except it covers the whole foot and is a lot thicker. And to make sure it stays on their food, which again is carrying all that weight on one middle toe per leg, the hoof interdigitates with the skin underneath. And these interdigitations have interdigitations. Think of it as Velcro, and the Velcro also has Velcro. When the horse is healthy, this system works great. But let's make something go wrong. Maybe there's too much weight on the hoof. Maybe the horse is septic. Maybe there's too much sugar, or insulin resistance. Whatever happens, the tissues in the hoof get inflamed and swell up. And because the hoof itself is there, there's nowhere for the swollen soft tissues to go. So the laminae get crushed, and you lose the support system that's holding the entire foot up. This is incredibly painful, and has to be caught early. Because if you let it go on too long, their toe bone will start to rotate because there's nothing holding it in place anymore (this is founder). And in some cases, the toe bone can actually fall through the bottom of the hoof.

TL;DR: Horses are actively trying to die on us.

Source: I'm a veterinary student.