After looking at various evolutionary points between the proto-hippo ancestor of horses and modern horses there was a point between “Hippo with fingers” and “fused hoof” where instead of splaying out to form a hippo foot or narrowing into a horses hoof, it could have gone horrifically wrong.
If the hoof fused, but only at the phalanges, the finger bones, it would create the little dainty hooves we draw, explain the awkward gait, and general clumsiness of fluffy ponies. the not quite fused phalanges would also need a flexible leathery pad to the hoof more like a hippo or elephant than a cow or horse’s hoof.
So to create the cutest little hoofsies, you take the worst elements from the horse’s evolution, and create a limb that is weak, unstable and easily broken.
