The first problem is that the size of a human baby's head is one of the biggest issues in reproduction. Even though a horse's head is composed of 34 bones which are not rigidly connected initially, allowing some flexibility during birth, it is still so much larger than a human head that a reverse centaur baby could not be delivered naturally.
The second issue relates to a horse's eyes. Horses have really big eyes designed to give a wide field of monocular vision. These eyes have evolved because horses are:
- prey animals that need to be able to detect threats over a wide field of view simultaneously; and
- really fast runners that need to be able to see where they are going, obstacles in the way etc.
What these eyes are not good at is getting a good look at things close in front of them. They are also the equivalent of red-green colour-blind. The second quality is a relatively minor disadvantage, but the lack of good binocular vision of close objects to the front is an overwhelming disadvantage for tool users.
Finally, the large jaw and tooth structures are designed for an animal that needs to crop vegetation from the ground. These are an unnecessary biological cost to a tool user with opposable thumbs, that can use its hands to obtain food, prepare it and convey it to its mouth.
In short, this creature would not evolve - the horse characteristics would only be of benefit if the rest of the creature changed so that it was a much faster runner that could not use tools (ie a horse), but the "unable to give birth" characteristic makes it an evolutionary non-starter. Given sufficiently advanced technology it might be possible to genetically engineer such a creature in an artificial womb, but even if a breeding population were established they would not be able to give birth without technological assistance - and as mentioned, their rubbish short-range forward vision would make them poor surgeons when conducting the required C-sections.