What injuries would this poor animal have - odds of survival?

There will definitely be soft tissue injury and could be dislocation of joints or fracture of bone, and the degree would depend on a number of factors. Whether the horse was hit from the side with impact to a heavily muscled area or from the rear with impact directly on a joint such as the hock with little soft tissue protecting it would make a difference. The upper leg has some muscling and some horses have more than others, which could absorb more of the impact. If the horse was already running in the direction of the force of impact, that force would be lessened. The width of the bumper and amount of surface area on the horse that absorbed the impact would make a difference, and even the state of relaxation or tension of the horse's soft tissues at the time of impact could make a difference. If tissue was torn open and exposed to the environment, then more body immune resources would have to deal with that and could increase the overall impact as well. Even at ten miles an hour or less, considerable damage can be done to the tissues by a direct hit from a vehicle. The fact that the horse was able to flee the scene is a good sign, but it is not a guarantee that serious injury didn't occur. Hormones and other chemical susbtances released in such a situation can completely mask pain while the "flight" response overcomes all other reactions in the body. So, it is really impossible to venture a valid guess as to what degree of injury may have been sustained by the horse.