Himalayan salt is a pink-hued salt that comes from the Punjab region of Pakistan. As any other salt, its major composition is sodium and chloride. The pink hue is a result of a higher concentration of iron in it, though not high enough to impact the horse’s diet – 39 parts per million, or mg/kg (i.e. if your horse ate 10g of it, he would take in 0.39mg of iron, while an idle horse needs 400mg of iron).
Because Himalayan salt is unprocessed (unlike table salt), it also contains a plethora of other minerals, some good (zinc, calcium) and some bad (arsenic, uranium, thalium). Most of these are in minute quantities so they won’t hurt your horse, but also won’t do much to supplement the diet.
Other types of rock salt (such as those from Utah or other areas of the Rocky Mountains) are similar to Himalayan salt in that they also contain other minerals.