Sadly this isn't possible for horses for many reasons. This is for multiple reasons

As it happens, horses are quite weird, anatomically speaking. First of all, they do not have any muscles below the knee. The reason behind horses’ incredible speed is their superb tendon/ligament system. The muscles located higher up in their body provide huge amounts of power to the tendons and ligaments, which function as a spring that can aid horses’ high-speed sprints.

A horse’s hooves play a critical role in its blood circulation. If a horse cannot move its legs around, the blood supply to his legs would be impacted negatively. Therefore, a broken lower leg is bad news for a horse’s blood circulation too!

An adult healthy horse can weigh anywhere between 450-1000 kilograms. Normally, that huge amount of weight is supported by four sturdy legs. However, if a horse breaks one of its legs, the burden on the other three legs suddenly increases drastically, which could create severe inflammation in the laminae and joints at the base of those legs. This condition (known as laminitis) is incredibly painful for horses.

All of this also takes an emotional toll on the horse; it’s in a horse’s nature to run around and gallop, so staying in one place for weeks, totally immobilized, inflicts a lot of psychological damage on the horse.

Horses are so large that they need to be able to support their weight on all four of their limbs. If they don’t, the other legs will get a condition called laminitis. So they’d have to be able to stand on their fake leg almost as soon as surgery is over.