On the battlefield, Catesby calls out to Norfolk for help, as Richard seems to have taken to the battlefield in mad desperation.
Richard's horse has been killed, and Richard is wandering the battlefield on foot, killing everything in his path with a fatalistic fury.
Richard yells out the famous line, "A horse, a horse! My kingdom for a horse!"
Richard is mad with bloodlust. He says he's slain five of the six "Richmonds" on the field, but not the Richmond he wants. (It was common practice to dress a few guys up as the leader of the troops to distract and serve as decoys. Swell job, huh?)
Richard runs off again, looking for horses and Richmond.