The Gospel Animals of Mark the Lion, Matthew the Man, John the Eagle, and Luke the Ox

As for the likeness of their faces, all four had the face of a Man, and the face of a Lion on the right side, and all four had the face of an Ox on the left side; they four also had the face of an Eagle.

- Ezekiel 1:10

The first Living Creature was like a Lion, the second was like an Ox, the third had a face like a Man, the fourth was like a flying Eagle. 
 

- Revelation 4:7

For several mysterious reasons, the early Church Fathers deliberately identified all four authors of the Gospels with the four Heavenly Cherubim originally described by the Prophet Ezekiel who each had four different faces resembling a Man, Lion, Eagle, and Ox. These same quadruple-faced entities were also spotted by the Apostle John during his prophetic visions as recorded in the Book of Revelation. Traditionally, these odd-looking, mysterious four-faced humanoids are referred to by most English-language Bibles as the 'Four Living Creatures'. Since around the 3rd century AD, the Christian Church has matched the Lion with Mark, the Man with Matthew, the Eagle with John, and the Ox with Luke. This can be seen as follows:

St. Mark: Winged Lion, The Resurrection.- The Lion was the symbol of St. Mark, who opens his gospel with the mission of John the Baptist, "the voice of one crying in the wilderness." He also sets forth the royal dignity of Christ and dwells upon His power manifested in the resurrection from the dead.. (Sacred-texts.com)

 St. Matthew: Winged Man, Incarnation.- To St. Matthew was given the creature in Human likeness, because he commences his gospel with the human generation of Christ, and because in his writings the human nature of Our Lord is more dwelt upon than the divine. (Sacred-texts.com)

 St. John: The Eagle, Ascension.- The Eagle was allotted to St. John because, as the eagle soars towards heaven, he soared in spirit upwards to the heaven of heavens to bring back to earth revelation of sublime and awful mysteries. (Sacred-texts.com)

 St. Luke: Winged Ox, Passion.- The form of the Ox, the beast of sacrifice, fitly sets forth the sacred office, and also the atonement for sin by blood, on which, in his gospel, he particularly dwells. (Sacred-texts.com)

Independently of their reference to the four evangelists these figures (living creatures) sometimes refer to the Incarnation, the Passion, the Resurrection, and the Ascension.

- Emblems of the Four Evangelists, Fictitious and Symbolic Creatures in Art (1909), by John Vinycomb (Sacred-texts.com)

I. St. Mark's Symbol of the Winged Lion represents the Resurrection of Christ..
II. St. Matthew's Symbol of the Winged Man represents the Incarnation of Christ.
III. St. John's Symbol of the Eagle represents the Ascension of Christ.
IV. St. Luke's Symbol of the Winged Ox  represents the Passion of Christ.