It is possible he did not exist.
And why was he pictured with a dog's head?
If you know one thing about St. Christopher, it is the story that he helped carry a child across a river, only to find later that the child was Christ. The fact that his name actually means "Christ-bearer" suggests that this was a made-up example of Christian charity. (Consider as well the story of Jesus on his way to Calvary and the woman who wipes his face, only to find his image on the cloth; her name is Veronica, "true image.")
There was supposedly a martyr named Christopher who was killed in either the reign of Emperor Decius (249-251) or Emperor Maximus Daia (308-313). The fact that churches were dedicated to him does not prove his historicity. Pope Paul
But why the dog head? Well, a thousand years after the martyr first lived, the Legenda Aurea (The Golden Legend, a book of saints' tales) says he was a Canaanite originally known as Reprobus (again, a suspiciously apt name meaning "rogue" or "unprincipled person"). Reprobus strove to serve the mightiest lord available, and even served the devil, until at last he found Christ and converted. Then...
The pagan king of Lycia took him as a fool and beheaded him after torturing him. Before his ordeal, however, St Christopher instructed the king to make a little clay mixed with his blood to rub on his eye (which was blinded by an arrow that had been meant for St Christopher). The king did as he was told and said, ‘in the name of God and St Christopher!’ He was healed immediately and was converted to Christianity. St. Christopher performed his miracle in martyrdom. [link]
And this is why he is depicted with a dog's head in the Eastern Orthodox tradition, because they mis-read "Canaanite" for canineus (canine), and assumed he was a member of the race of Cynocephali, the dog-headed people.
And we will learn about them tomorrow.