I get those books confused as well but I think you are correct, the horse rearing up in the open topped trailer and creaming it's head on the gate was My Friend Flicka. Isn't it funny how all these horse books all follow the same story line? Maybe that's why we liked The Black Stallion books so much, they were upbeat and fun.
Boy meets horse, boy loves horse, boy loses horse, boy finds horse . . . LOL!