Ask Mr. Smarty Plants

Here is what the article "Poisonous Plants" by Willis Lamm of Trail Blazer Magazine says about choke cherry and wild cherry (Prunus):

"Chokecherries, growing in bushes up to 12 feet high, are popular for their jelly producing berries. They are common throughout the US, often found along roadsides or creek bottoms. Unfortunately the leaves, which are particularly toxic when stressed or wilted, as well as the bark from chokecherries and wild cherries are cyanide producing.

Death in horses can occur literally in minutes after the horse has ingested the leaves. The horse will appear to have trouble breathing, show flared nostrils and lose bowel and urinary control. Lack of coordination and trembling may also appear, along with agitation. A severely poisoned horse will drop to the ground, kick a few times, then die.

Poisoned horses can be saved, however usually veterinary help cannot arrive in time as the effects of cyanide poisoning progress rapidly."

The Poisonous Plants of Pennsylvania has more information about the chemicals responsible for Prunus spp. toxicity. Other toxic plant databases (Poisonous Plants of North Carolina, Cornell University Plants Poisonoous to Livestock, Canadian Poisonous Plants Information System) also list various Prunus spp. as poisonous.

From the ASPCA you can read, Toxic and Non-Toxic Plant List – Horses, that gives toxicity information about many native and non-native plants.