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
Depends on the locator. The Garmin Astro that I have doesn't use cell, and works where there is no cell service at all (which is most of the places I ride). It does work a lot better if you have a line of sight between you and dog/horse.
our horses tie for days with no issues, it's when out on the trial that would worry me.
Around here my horses are 100% safe however when hundreds of miles from home and suddenly riding in an area where bear and mountion lions live. I don't expect to encounter one but even the smell to an unfamiliar nose could be enough to spook a horse
Dare say there are others with more experience, but I have encountered both bears and (one) mountain lion while riding, with no real spooking, just nervousness. Deer bounding away, or (as yesterday) ducks suddenly taking off from a small pond... It seems to be more of a startle reflex from something unexpected, than actual/apparent threat.
I've been trying for over a year to build a GPS transceiver to embed in halters and bridles, but either it's too wide or not flat enough. Been using open source components in case it works out, then I could patent it.