The horse impacted the surplus of food and population because it allowed more time to hunt, less people got killed in war, and the horse could have brought diseases that the aponent wasn't adapted to, and the aponent's soldiers could die. Calvary didn't give nearly as many risks of dying as fighting on the ground. All these reasons helped the conquistadors battle with the incas.
Horses affected weapons because they were the red of the purple, horses made it so that the soldiers carrying the weapons could have a strong and successful angle from above. Without horses put with the weapons, the conquistadors wouldn't have had as good of a chance of coming out on top of the battle against the incas.