The evangelists are the 'authors' of the Gospels (Matthew, Mark, Luke, and John). They were often depicted as scribes from the Early Christian period on. The rendering of their figures is often accompanied by, or conflated with, the Evangelist symbols. Other types of depictions of the evangelists include scenes of Saint John on Patmos (found in manuscripts of the Book of Revelation, or Apocalypse) and of Saint Luke making a portrait of the Virgin and Child (an image ultimately of Byzantine derivation, which is encountered most often in Books of Hours).
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).