It seems any snake (even non-venomous snakes) horses don't like them. I don't think a snake could really hurt a horse except for a bite, because a snake couldn't pump enough venom into a horse to kill it. Why are horses afraid of snakes?
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Horses and other hoofed animals, such as deer, are instinctively scared of snakes, and they try to eliminate the threat by stomping on the serpents. These animals do not know how to tell a venomous snake from a harmless one, but a venomous snake can cause great bodily harm to a horse. Even if the snake does not kill the horse, it can cripple one, as the venom of rattlesnakes destroys tissues. A horse or deer with a bum leg or two will fall prey easily to a predator such as wolves, bears, or mountain lions.
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Everybody is afraid of snakes and horses are no exception. The body type signatures of snakes and spiders are feared by almost all mammals. There has been some interesting research on why this is. Current thinking is that the mammalian nervous system is designed to pick up certain styles of movement and shape - perhaps because it might be food but more likely because it might be dangerous. The fear response is innate (not from experience) and not from a rational assessment of actual risk. All mammals originate from little mouse-like shrews that probably had much to fear from snakes and spiders. Maybe that is why we worry even if we know that there is no risk.