In life, Guinefort was a greyhound.
The story is brought to us by Dominican monk Stephen of Bourbon (1180 - 1261) in 1250. A knight in Lyon left his baby to go hunting, secure that his faithful dog, Guinefort, would guard the child. When he returned, the dog's mouth was all bloody, the room a mess, the crib overturned, and the baby nowhere to be found. In a fit of sudden rage, the knight cut off Guinefort's head. Hearing a child's cries, they discovered him under the cradle, surrounded by the bloody body of a viper with dog bites all over it. Regretting his killing of Guinefort, they made a grave of stones surrounded with a grove.
Locals, learning of the dog's actions and martyrdom, visited the site more and more. It became a place to bring ill children, or mothers worried about the outcome of their pregnancy, to pray for the intercession of "Saint" Guinefort.
Finding this, Stephen of Bourbon records that he had the dog's bones disinterred and destroyed; the trees were burned down. He shows sympathy for the heroic dog so unjustly killed, but a dog saint? Not on his watch!
It was not allowed for a dog to become a saint, but was it possible for a saint to be a dog? Let me tell you about St. Christopher (yes, that one) tomorrow.