When horses are exercised, fluid loss occurs in the form of sweat. Horse can cope with a small loss of fluid say 5%. After that the skin will become less elastic indicating dehydration. Adding electrolytes to water will help prevent dehydration. Electrolytes are salts  that dissociate into separate ions when they are dissolved in water. Electrolytes are integral to nerve and muscle function, as well as to almost every other physiological function in the body.