This painting by a Cologne artist, together with 'The Mass of Saint Hubert' was one of the inside shutters of an altarpiece from the Benedictine Abbey at Werden, near Cologne. The outside shutters, depicting various saints, are also in the National Gallery Collection.
A stag with a crucifix between his antlers is said to have appeared to Hubert (about 656 - 727) while he was hunting on a holy day, after which he adopted the religious life, eventually becoming Bishop of Liège. The same story was told of Saint Eustace.
From The National Gallery Podcast: Episode Thirty Seven, November 2009