In the Middle Ages, symbols evolved to represent each of the four Gospels of Matthew, Mark, Luke and John. Matthew's symbol was an angel, or winged man, because his Gospel begins with the human genealogy of Jesus. Mark's symbol is a lion because his Gospel begins with John the Baptist, whose preaching is described as a voice crying in the wilderness like the roar of a lion. Luke's Gospel begins with the story of Zacharias making sacrifices in the temple, and since oxen and calves were often used for sacrifices, Luke's symbol is an ox or a calf. And John's symbol is an eagle because his Gospel begins with Jesus existing in heaven as "the Word of God" before he came to earth.
All of these symbols are described in a vision of Ezekiel.
The image shown here is a detail from a carpet page of the Book of Kells. You can get a closer look in our Book of Kells Image Gallery. You can also find out more about these symbols through the pages for the Gospels in the fabulous Les Très Riches Heures du Duc de Berry:
The image from the Book of Kells is in the Public Domain.