Peach trees are poisoness to horses because of the leaves and also when the fruit is ripe it can also cause colic. The reason the leaves are poisioness is because the leaves can produce something called cyanide. This is produced by the leaves when they are wilted or damaged. The results can be fatal in minutes if enough of it is eaten.
Lack of coordination
Cyanide poisoning is curable, however, it is usually impossible to get a vet to the horse quickly enough to save him. This is because the effects happen so quickly.
Cyanide is lethal because it reacts with iron in your horse's body to stop cellular respiration. Cyanide also prevents oxygen from being transported via the blood stream, so the cause of death is suffocation due to lack of oxygen.
Artical: Peach Tree: A Poisonous Plant for Horses