Goats are one of the most common four-legged barn buddies for horses, but veterinarians continue to caution that some goat feeds contain an ingredient that can be lethal to horses. Rumensin (monensin sodium) is often included in commercial feeds designed for ruminants like cattle and goats. Although actually an antibiotic, when fed continuously in low doses Rumensin promotes weight gain and milk production in goats. But in equines the product damages heart muscle and can lead to cardiac failure. Horses that do survive chowing down on feed containing Rumensin are usually so debilitated that they can never resume normal activities and may require euthanasia. Therefore, unless you are absolutely certain that the feed you’re offering your goat is free of Rumensin, keep the goat grain far, far away from your horse.