Poseidon- many people have debated the idea of there being toxic amounts of copper in goat feed, fine for goats- but lethal to horses. Really though, there is often more copper found in general horse feed such as Purina horse and pony feed than in goat feed. However, you're right. Most people say not to chance it with the horse by letting it get into goat food, not only because of the copper but because goat feed generally is made up of only corn, oats, and barley, all things that can be fed in limited amounts to horses, but can make any horse sick if too much is ingested. I would personally recommend just buying a nice, hardy goat who does well on roughage, so you dont have to worry about that. And if you do decide that the goat needs something extra, they do perfectly fine with generic horse and pony feed (thats what our goats eat, because they have a limited amount of roughage for both the horses and goats). Generally goats are much more hardy than horses are, so changing their food is a better idea than the horses.