In traditional iconography, the Evangelists (the writers of the four Gospels) are often depicted as angelic creatures, based on the four living creatures around the throne of God in the books of Ezekiel and Revelation. Saint Luke is depicted as a Bull (or an Ox).
In the midst [of the cloud], there was as it were the likeness of four living creatures. This was the likeness of their faces: the face of a man, the face of a lion on the right side of the foursome, the face of an ox on the left, and the face of an eagle. (Ezekiel 1:5, 10)
This icon was painted by St. Andrei Rublev for an illuminated Gospel book.