Here in MI, I've read that the two worst poisonous to horses are Red Maple and Cherry (any species).
Apparantly, 3 cherry leaves will kill a horse before the Vet can get there - a friend of a friend had the horrible experience of seeing this happen to her beloved horse. Cyanide is the culprit, and the worst is in the fall when they are concentrated with it!
When I read that, I had a panic attack noticing 3 'evil' cherry trees well behind our newly laid pasture fencing - not far enough in my opinion with wind in this elevated area! Needless to say, they were cut down immediately!!!!
I sleep well at night knowing she's safe....
I am a Naturalist and know a good amount about MI trees, but also as a first time horse owner, was so glad to have learned this information!!!!!