Horseshoes

In the wild, horses wear down their hooves as fast as they grow them. But domesticated horses, with the extra weight of rider and equipment, will wear away hooves much more quickly. Without protection, the horse would quickly become lame and useless.
Metal horseshoes are used to counteract the wear and tear on the mount's hooves. The horseshoe is nothing more than a steel bar hammered into a "U" shape, then nailed tightly into the hoof of the horse. The nails cause no discomfort to the animal. With horseshoes, the mount can travel farther and faster.
Throwing a horseshoe or getting a stone lodged between the shoe and the hoof are events that need extra attention if the horse is to be kept in good condition.

Magical Horseshoes

Any number of different types of magical horseshoes can be added to the game; those that allow unusual speed, the ability to move on water or through the air, those that act as magical weapons, and so forth.
Note: horses are not the only work animals that are in need of shoeing. Mules, donkeys, asses, and oxen also need care for their hooves. The cost is the same no matter the animal.