Thar's Horses in them thar woods.... or not?

This is a question that comes up surprisingly often.

The answer is .... 'it depends'.

And it depends on a lot of things. Horses are grazers, different than browsers like deer, or even donkeys. In addition to the nutrition (calories and vitamins and stuff), they need the long stemmed grasses in order for their guts to function properly.

Can they live without grass or hay (such as in the woods)? Sure. For while. Some horses will be very picky about what they eat, but most would put a goat to shame before they starved to death. They would start to eat odd plants, anything leafy, bark, sticks, even the old layer of detritus on the forest floor (one of my mares ate a mosquito dunk today (and she is decidedly not starving)) - and if they didn't get impacted, colic and die from that -- they would eventually be dealing with the consequences of long term malnutrition.

But depending on the climate, your woods may have lovely meadows. Easily enough to support a horse or two for a short period of time. And another meadow down the way. But not a herd of horses.

Also consider, if you're writing a horse that's been in the woods for a long time, it's likely to have life-long issues stemming from that experience. Anything from degenerative joint disease, if it was a young foal, if the woods-horse was a mare carrying a foal, or with a very young foal at her side, the foal could lack of integrity in it's soft tissues, ligaments and tendons, poorly laid down cartilage and even some amount of malformation of the delicate bones in the knees and hocks. These things could cause the young horse a lot of pain as he matures, especially when he comes of riding age and is put to work.

An older horse could have internal issues, anything from something as simple as a stick lodged in their mouth that festers, to internal damage that contributes to recurrent colic.

He may have developed a taste for toxic weeds if he managed to eat enough in the woods to keep him alive, but not so much it killed him.

And it's more than just nutrition: A horse's most basic self-preservations instincts are compromised in the woods. They are herd animals who find safety and security in numbers (and remember a real herd can't live in most woodland areas, so they'll be searching for other horses.)

Their eye-sight is designed for the open ranges. They see short distances with their heads down and long distances when their heads are raised. Aside from their keen sense of hearing, their eyes are their early warning system. But you can't see very far in the woods.

Their first line of defense is to run. And they can't run as fast in the woods, not if they're looking for a way through and having to zig-zag, dodging trees. So most horses, simply wouldn't stay in the woods. If they were chased into the woods, or abandoned there, they would travel, snatching what they could along the way, maybe stopping for a few hours at a creek or a meadow, but then they would move along - looking for food, but more dramatically, looking for safety. Although there are always exceptions. We had a local case of a horse that had been abused, rescued and (mostly) rehabilitated, getting loose and running to hide in the woods, and no, he didn't survive.

If a fully tacked horse gets loose from a rider, there is additional danger of the reins, or even the saddle, getting tangled in vines and holding the horse in place until he dies of dehydration. Another main consideration is that the horse may eventually have tack galls that fester.

But one of the main things authors need to remember is that it's not really natural for horses to be in the woods, that there needs to be a good reason for the horse to be there -- the worldbuilding has to be solid enough that the reader sees what the horse is going through trying to find enough forage -- and there needs to be appropriate consequences for the length of time the horse was in there.