Emphasyma (in humans) is from second hand cigarette smoke. If a horse has emphasyma it may be from inhaling smoke over a long period of time. But what is likely the case is that the horse has what is...
The line is from Shakespeare's play, Richard III (Richard the third). In the middle of a great battle, King Richard has lost his horse and finds himself afoot. In desperation, he cries out, 'A horse!...