Sole is the natural protecting
Feet are often very hard and contracted in a dry environment. Horses have often retained a lot of sole. This is a natural protection against the harsh environment that they are in. Take care not to remove too much of the protective element.
Normally in a dry environment, horses feet contract from the norm. This is perfectly natural. The hoof wall moisture content varies from 25%-30% in normal conditions. In a dry environment, this moisture content CAN get as low as 15%. This makes the very hoof extremely hard and often brittle.
Ensure your tools are sharp!!! Blunt tools will not be able to trim the dried horn. When selecting horseshoes also think about your nail selection, ideally something slim in the blade but strong to drive (MX Endura). A wide web shoe will give maximum support, cover and durability against stones and rocks, but also a wide web shoe will protect against the horse's feet sinking into a soft surface such as sand.