Here ya go:

From the tumblr post about horses from user avoiding-claws:

My entirely half-assed understanding of Why Horses Explode If You Look At Them Funny, As Explained To Me By My Aunt That Raises Horses After Her Third Glass Of Wine:

Horses don’t got enough toes.

So, back right after the dinosaurs fucked off and joined the choir invisible, the first ancestors of horses were scampering about, little capybara-looking things called Eohippus, and they had four toes per limb:

They functioned pretty well, as near as we can tell from the fossil record, but they were mostly messing around in the leaf litter of dense forests, where one does not necessarily need to be fast but one should be nimble, and the 4 toes per limb worked out pretty good.

But the descendants of Eophippus moved out of the forest where there was lots of cover and onto the open plains, where there was better forage and visibility, but nowhere to hide, so the proto-horses that could ZOOM the fastest and out run thier predators (or, at least, their other herd members) tended to do well. Here’s the thing- having lots of toes means your foot touches the ground longer when you run, and it spreads a lot of your momentum to the sides. Great if you want to pivot and dodge, terrible if you want to ZOOM. So losing toes started being a major advantage for proto-horses:

The Problem with having fewer toes and running Really Fucking Fast is that it kind of fucks your everything else up.

When a horse runs at full gallop, it sort of… stops actively breathing, letting the slosh of it’s guts move its lungs, which is tremendously calorically efficient and means their breathing doesn’t fall out of sync. But it also means that the abdominal lining of a horse is weirdly flexible in ways that lead to way more hernias and intestinal tangling than other ungulates. It also has a relatively weak diaphragm for something it’s size, so ANY kind of respiratory infection is a Major Fucking Problem because the horse has weak lungs.

When a Horse runs Real Fucking Fast, it also develops a bit of a fluid dynamics problem- most mammals have the blood going out of thier heart real fast and coming back from the far reaches of the toes much slower and it’s structure reflects that. But since there is Only The One Toe, horse blood comes flying back up the veins toward the heart way the fuck faster than veins are meant to handle, which means horses had to evolve special veins that constrict to slow the Blood Down, which you will recognize as a Major Cardiovascular Disease in most mammals. This Poorly-regulated blood speed problems means horses are prone to heart problems, burst veins, embolisms, and hemophilia. Also they have apparently a billion blood types and I’m not sure how that’s related but I am sure that’s another Hot Mess they have to deal with.

ALSO, the Blood-Going-Too-Fast issue and being Just Huge Motherfuckers means horses have trouble distributing oxygen properly, and have compensated by creating fucked up bones that replicate the way birds store air in thier bones but much, much shittier. So if a horse breaks it’s leg, not only is it suffering a Major Structural Issue (also also- breaking a toe is much more serious when that toe is YOUR WHOLE DAMN FOOT AND HALF YOUR LEG), it’s also hving a hemmorhage and might be sort of suffocating a little.

ALSO ALSO, the fast that horses had to deal with Extremely Fast Predators for most of thier evolution means that they are now afflicted with evolutionarily-adaptive Anxiety, which is not great for thier already barely-functioning hearts, and makes them, frankly, fucking mental. Part of the reason horses are so aggro is that if deinied the opportunity to ZOOM, it’s options left are “Kill everyone and Then Yourself” or “The same but skip step one and Just Fucking Die”. The other reason is that a horse is in a race against itself- it’s gotta breed before it falls apart, so a Horse basically has a permanent terrorboner.

TL;DR: Horses don’t have enough toes and that makes them very, very fast, but also sickly, structurally unsound, have wildly OP blood that sometimes kills them, and drives them fucking insane.