The hoof is designed well enough to support the frame of a horse and allow movement. But it fails in other ways. Just because a mechanism such as a hoof is the culmination of many thousands of years of evolution, does not mean it is perfect and could not be improved with a little assistance from humans.
Coat Shedding
It evolved that horses are more comfortable at colder temperatures than warmer ones. Their thermal neutral temperature range varies between 5 and 12 deg C. Whereas humans have a comfortable temperature around 20-22 deg C. I won’t go into how this works, but the way a horse’s coat grows is part of the mechanism. It also helps that horses primarily breathe through their nose and not their mouth when it comes to reducing heat loss.