War Horse by Michael Morpurgo.
War Horse (1982) is told entirely from the point of view of Joey, a horse born on a Devon farm and sold to the British Army at the start of the First World War. Joey serves on both the British and German sides, and survives remarkable experiences — from cavalry charges to pulling ambulances in No Man's Land. Eventually he is reunited with his original owner, Albert, after an extraordinary search through the devastation of the Western Front.
Joey is bought as a foal by an alcoholic Devon farmer, but is cared for by the farmer's teenage son, Albert, who trains him with great patience and love. When the family falls into debt and the First World War begins, Joey is sold to Captain Nicholls of the British cavalry. Albert is too young to enlist but promises to find Joey whatever happens.
Joey experiences the war from multiple perspectives: he serves under Captain Nicholls (killed in an early cavalry charge), then is captured by the Germans and used as a farm horse by two young German soldier-brothers, Friedrich and Karl. Later he is used to pull ambulance wagons on the Western Front, and finally ends up in No Man's Land, tangled in barbed wire. A British soldier and a German soldier cooperate in a truce to free him.
Joey is auctioned at the war's end. A French farmer nearly buys him, but Albert — who has survived the war, despite being temporarily blinded by a gas attack — recognises Joey and buys him back. They return to Devon together.