This paper addresses the question of sources for the idiosyncratic representations of the Evangelists created by ninth-century Breton monks and explores what might have prompted the occasional substitution of the horse for the traditional symbol of Saint Mark, the lion. Challenging the assumption of a “Celtic” connotation of equine imagery, this study suggests that the monks were directly influenced by Gregory the Great’s allegorical interpretation of the horse given in the Moral Reflections on Job 39. Echoing Gregory as he draws a parallel between the emblematic qualities of the horse (strength, perseverance, courage) and those required of a servant of God, the innovative iconography of Saint Mark unites the lion and the horse in celebratory remembrance of Christ’s sacrifice and resurrection and glorifies in the image of the Evangelist the tireless preacher, the devoted and fearless Horse of God.