Horses have adapted by being very fast however that comes with fragility, without 4 legs horses break down very quickly, they structurally cannot survive this and the entire time they are alive they are in pain, even attempting prosthetics is a loaded topic in vet med.
for people unfamiliar with rocky: does this look like a functionally sound horse?
his leg was already buckling during his first week alive. he had to live over a year as he put more and more weight, causing even more stress on it. everyone involved should be investigated