Yes, but it is best to get the aspirin for horses from the Vet because they have larger doses in just one pill and they can tell you how much to give, safely.
It's probably not advised. I don't know much about it, but I know that it could be possibly dangerous. Try going to the veterinarian's office. The vet is more knowledgeable on this. I seriously wouldnt give a horse human aspirin. It's better to be safe, than to find that later the horse fell over dead because of it.