I have never used one for a horse, but they do sell GPS-enabled locators for dogs. Unfortunately in the reviews I have seen, they are expensive and not that great - short battery life and you are screwed if you are in an area with poor cell reception (IE most wilderness areas).
I would say it's a better use of your time and energy to make sure that your horse is secured so this is not a problem, and if they do get away from you that they are easy to identify as yours if found. A freeze brand, a microchip, and putting a dog tag with your name and phone number on their bridle or halter can all be useful.
Also, teach your horse to hobble if it doesn't already well before you go. Also, teaching them to ground-tie helps a lot :)