War Horse, Chapter 9

In Chapter Nine, a field doctor arrives and announces that Joey and Topthorn must be put to work pulling ambulance carts. Initially offended that such fine horses would be used for such rough labor, the tall German (whose name is revealed to be Herr Hauptmann) decides to oversee the work. Together, the horses save the lives of wounded soldiers, earning the field doctor's approval. The animals are then put in a stable—the first that Joey has been in since the war began—and are greeted by two new acquaintances during the night: an old Frenchman and his young granddaughter.

Note, Hauptmann wants to keep Joey and Topthorn for his Regiment of Lancers.