The Butterfly Lion by Michael Morpurgo

Millie meets a young boy who has run away from his boarding school. To pacify him she tells him the story of a brave man named Bertie and the butterfly lion; a white lion that he rescued as a cub when he was a child, but was separated from upon being forced into boarding school. His lion was sold to the circus, but Bertie promised that one day, they would reunite.