The phrase is repeated ironically, when someone needs some insignificant item. Shakespeare shows that the value and importance of things may change suddenly and simple and unimportant things, like a horse in the battle, could become important than a whole kingdom. The sense in this line is ironic, as someone wants something insignificant to complete an important task. The king here means that if he does not find his horse, he may lose his kingdom, because in that case he would be either killed or face defeat.