I have never seen the reaction a horse has to cyanide poisoning but I have seen what it does to a dog. Years ago my daughters lab was fine one minute 20 min later she was circling around over and over, falling down, puking, the whites of her eyes were blood red. I rushed her to the vet and the first thing he asked was "do you have a peach tree?" then explained cyanide poisoning. If they make it the first 45min their prognosis is good at least for dogs, I don't know what time frame you have with horses.. 24hrs and 300$ later our dog was perfectly fine. The tree was my neighbors but it hung over our fence. Needless to say I cut all the limbs off on my side of the fence line and made sure there were never any fallen peaches in my yard as my vet told me the peach seeds also contain cyanide.