Who is Topthorn? How is Joey like him?

They are both strong and fast. Nicholls personally takes Joey into final maneuvers, where Joey meets the black stallion of Captain Jamie Stewart, named Topthorn. Joey and Topthorn prove themselves to be the fastest horses in the regiment, with Joey beating Topthorn during a charge maneuver. The two become good friends. The next day, the regiments ships out for war by boat.