I haven't kept a horse in the past 10 years, but before that time I always kept two or three horses. In my opinion, horses are a disaster looking for a place to happen. Horses are a large animal veterinarian's dream animal. Cows scratch their heads on everything. The sharp angles of a beehive would be very attractive to a cow. But, in my area, we have a large population of feral pigs, and I am sure they would destroy a bee colony and eat the honey and the bees. They have long hair and tough skin, and I don't think a bee sting would be much of a deterrent. Therefore, I keep my bees behind cattle panel fences.
In brief, in my area, I think bees should be fenced away from all other livestock.