How do horses mouth help it adapt to its environment?

Hoses in the wild often travel large distances in herds, so indifferent area's there are different plants to feed on which usually have different textures. To feed a horses mouth will get stronger or weaker depending on what it has been eating. And to answer your question say a horse is unable to eat a certain plant because it is to hard on the mouth well what else is it going to eat? So it would starve so If its mouth adapts to eating hard plants then it wouldn't starve.