Are holly trees toxic to horses?

Holly has an array of potential toxicants, but its potential to cause symptoms is quite variable and is highest in fruits when consumed at 0.28% of body weight if fresh and 0.71% if dried. In most instances of intoxication, the signs will be indicative of mild to moderate dysfunction and may include diarrhea, which lasts only a few hours. A lethal outcome is not likely. (From Toxic Plants of North America, Burrows & Tyrl)