A Horse called Hero by Sam Angus

It is 1940. As the Second World War escalates and London becomes a target for German bombs, Dodo and her horse-mad little brother Wolfie are evacuated to the country, away from everything they know. After weeks of homesick loneliness, they come across an orphaned foal. They name the horse Hero for surviving against the odds and together they raise him, train him, and learn to ride. Their days are suddenly full of life and excitement again, but the shadow of war looms over their peaceful existence, and soon Hero must live up to his name...