All's well that ends well! I am so happy to hear that you're pretty sure the mystery plant was the culprit & that the horses are healing.
This thread has really opened my eyes. I had no idea so many plants could be toxic to some degree or another to livestock. I knew about wild black cherry but not about anything else. Not that I have livestock or am ever likely to but I do have neighbors with livestock and I would not wish to plant anything that might get into their pastures and harm their cows or horses or donkeys.
I am a strong believer in the simple fact is that what matters in this life is how we treat others. I think that's what living is all about. Not what I've done in my life but how I've treated others. ~~ Sharon Brown