Fractured bones in horses are thankfully not too common. However when they do occur they are potentially catastrophic. Depending on what kind of fracture has occurred and where in the leg it is will depend on how lame the horse is and how distressed it is. Unfortunately, unlike in small animals and people, horses are not very good candidates for fixing fractures.
In the factsheet are some x-rays of various fractures that have been seen at the practice.