War Horse by Michael Morpurgo

Joey, a young farm horse with a strong bond to his owner's son Albert, is sold to the army. Distraught, Albert tries to join the army to be with his horse, but is told he's too young and promises he will reunite with Joey some day. Within the war, Joey survives great peril and loses many companions, riders and other horses alike, but holds out hope of seeing Albert once more.