Mothballs r TOXIC to horses, humans, dogs, cats, etc.
Master Gardener Jerry Baker recommends mothballs as deterrent for snakes & cats. Because of the smell it is unlikely that anything would ingest them, however, I am paranoid. I do not want to cause harm to any animal just to ward off a snake.
I put them around a small inground pond once, b/c fish ARE on the menu for snakes.
I've never seen a snake leaving the area coughing & hacking from the smell, but for every snake u see, there is probably a dozen u never see.
Also, snakes r wonderful climbers, they're not just on the ground.
Is this a real threat or just ur fear of snakes? Snakes r active here most of the year. In 44 years I've only had one horse encounter a snake & get bitten. She survived without more treatment than a tetanus shot & some salve on the bite. Never missed a meal.
Snakes know they can not eat a horse, so even if bitten by a venomous species, 9 times in 10, they're not invenomated.
Snakes & horses have survived hundreds of thousands of years in the same habitat & neither has killed the other off.
I'd take that worry off my priority list