Does your horse get enough salt from a salt lick?
The system that manages salt intake is about regulating blood pressure. The brain doesn't look at salt in the muscles. When the horse sweats, it uses salt and electrolytes, so while the blood pressure and blood volume may be normal, the horse's system might be telling it that salt is fine, when it's actually deficient. If they have large, rapid sweat losses, the system that regulates salt might take longer to figure that out, because it's looking at blood pressure, not salt and electrolytes in tissues.
There is a persistent myth that horses regulate their own salt intake, but all horses like salt. They might eat more than they need because they like it, or because they are bored. As described above, horses in work with regular heavy sweat losses might eat less than they need. A horse in light work doubles their salt requirement. A horse in heavy work will need more. A rough guide is an extra 12g of salt per hour of work.
Horses without enough salt can cramp, dehydrate, colic, suffer from heat stroke and generally have poor performance.
Salt is cheap, and palatable. It is very easy to supplement. A good rule of thumb is a tablespoon a day and then an extra tablespoon for a horse in work, or wearing a rug. A horse in heavy work will need more again.
Timing is important when adding extra salt for a horse in heavy work. Extra salt given hours before work will be excreted in urine. It's best to give them extra salt about 30 minutes before work and then again afterwards.
You can get a Himalayan block, like the one pictured, but you'll need to weigh it to measure accurately how much they are getting. And you won't know in a shared pasture. Just adding pool salt to your horse's feed is a cheap and accurate way of ensuring horse is getting the salt they need.