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.
Many of the chemical analyses on Himalayan salt report that it does not contain any iodine. As such, there are goiter issues (swelling of the neck due to enlargement of the thyroid gland) as a result of iodine deficiencies in humans in those areas where it is mined and consumed readily. Iodized salt is normally the sole source of iodine for horses (and humans). Therefore, iodized salt blocks, or trace mineral salt blocks are generally recommended for horses. It should be noted that some types of other rock salts do contain some iodine, and that some sources of Himalayan salt appear to also have iodine. However, neither would have similar amounts to what is in iodized salt.
Some horses have a tendency to eat their salt blocks, and because Himalayan salt is quite hard, horses are generally unable to bite it (they can just lick it). In fact, a popular option for horses is to offer Himalayan salt on a roller, so it is a bit of a toy as well. Another solution would be mixing in loose (iodized) table salt into a feed.
As with any type of supplement, owners should ask for a full analysis of the product to ensure you’re getting what you pay for.