The most natural way to cool down a horse is to bring it back into the shade or stable and tie it there. The temperature will cool down slowly but naturally in the shade. Take the saddle off the horse. Use a curry comb to brush the matted areas of the horse's coat. Detangling the hair of the horse helps in getting air to the skin to bring down the temperature.
Another tip that is very effective is to dismount the horse at the end of the ride and to walk it slowly around before tying it down. This slow walk reduces the breathing and heart rate as it brings down the body temperature. Reduce the run to a trot and finally to a walk.
Drinking cool water also helps in reducing the body temperature. Place a bucket of cool water for your horse to consume. However, give the horse a minimal amount to drink in the start. Give it a break and then let your horse drink some water. If the horse is allowed to drink in excess right in the start, it will develop cramps just like humans do. Remember to keep a balance between the amount of cool water your horse drinks and the duration of a particular activity it is doing.
A simple and effective method is to turn on the garden hose and wash the horse with it. The water will help in taking the heat off the horse. If a hose is not available, buckets can also do the job. However, it is very important to remove the saddle from the back of the horse and replace it with a fresh shower of cold water. Allow the water to dry and after a while use the hose to put more cool water on your horse.