First, and most important your horse is dehydrated in most cases. Second, your animals stomach is in a highly acidic condition and almost empty. Third, the animals muscles are loaded with the debris of metabolism. Fourth, the animals glycogen reserves are depleted. Fifth, and most important, the animal is in electrolyte overload, because the percentage of body water lost is much greater than the percentage of body minerals lost.
Rehydrate the animal immediately by allowing him to sip plain cold drinking water. Sip, not gulp. Be patient, because the thirst response is still inhibited after performance. Avoid sugar/citric type of supplements in water at first, because they inhibit rehydration, and only add to stomach acidity, promoting cramps. Plain water is the only story.
Keep walking your horse after an event to cool him down. Muscle cramps and post-event injuries often occur because insufficient blood gets to the fatigued muscles to remove wastes. A lot of the force for blood circulation comes not from the heart, but from working muscles. Keep drinking and walking!
Continue to hydrate extra for the next twelve hours. A common problem among performance horses is chronic partial dehydration. The horse may not drink enough to completely hydrate and IV fluids may be considered.
Carbohydrates are equally important. You want to to replace muscle glycogen fast. So after the initial rehydrating period is complete, the animal can then be fed a easily digested complex carbohydrate meal and grass hay.