If a horse has been living with other horses, moving to live alone might be very stressful for her. Some horses react very badly to it - so it's not the bears that get them. It's their instincts and mind, and it can affect them physically, not only psychologically. As for wildlife, a really good fencing and a good hot wire should sort the problem, but is there any way you could get your horse a companion? A pony, a donkey, even a goat would do, although an equine is preferred.
I have come a long way, to surrender my shadow to the shadow of my horse.