Horses love to nibble
I have a small herd of beavers, at least they think they are beavers, they nibble on trees and bushes out in pasture. When we lived on a ranch that had no trees in the pastures, the horses would nibble on the wooden fences. Horses will also reach over fences to nibble on anything that interests them. I had a beautiful young willow tree that had been growing about 6 feet outside of our pasture fence. The willow tree was to be a shade tree for our deck that was another 15 feet from the tree. One day, Kid, one of our horses, just stretched out his neck and reached the trunk of the tree and snapped it in half.
The willow tree also serves as a reminder that anything located within reach of a horse can be damaged. This includes car mirrors, which often get chomped off, if the car is parked too close to the horses' pasture. This is something that your cheap car insurance will not cover. I was once at a boarding stable that the manager had to explain to visitors why they could not park next to the horses. Most of them were not aware of horses dismantling things within their reach.