Mark, the Evangelist, is symbolised by a lion, because he begins his gospel with the scene of John the Baptist and Jesus in the Wilderness.
Matthew, whose gospel commences with the humanity of Jesus as a descendant of David, is the only one of the Evangelists represented as a man.
Luke is symbolised by a bull or calf, and John by an eagle—the former because his gospel opens with the priests sacrificing in the Temple, and the latter because he soars high and begins his gospel with the divinity of the Logos.
In Greek and Roman art the lion’s head is used particularly on fountains. The Egyptians employed the lion, to symbolise the annual inundations of the Nile, which happens when the sun is in Leo.
The Serpent
The serpent in ancient times was symbolical of wisdom and subtlety, and, considered as a guardian spirit, is depicted on altars. It was also the symbol of Hygeia, the goddess of Health, from the tradition that Æsculapius assumed the form of a serpent during a pestilence in Rome.
In later art the serpent appears as a tempter. The Brazen Serpent of the Hebrews that gave newness of life to those who, bitten by the fiery dragon, raised their eyes to it, is an anticipation of the Crucifixion.
The Dragon
The mythical dragon is a Middle Age symbol of sin in general and Paganism in particular. The Celtic use of the word for “a chief” is the source of the legendary dragon slayer, as a knight killing a chief thereby slew a dragon.
The dragon, which appears as a guardian, as in the garden of the Hesperides, watching the tree bearing the golden apples of Hera, is also a poetic allusion to flood or inundation.