It was a sighthound or greyhound, owned by the lord of Thoire et Villars from the Auvergne-Rhône-Alpes region in France.
One day the knight went hunting and left Guinefort in the care of his son, who was resting in his cradle. When he returned he found the crib overturned and the dog with its mouth full of blood and he assumed that it had killed his son. He immediately stabbed the animal. Suddenly he heard a cry coming from under the crib. His son was alive and beside him lay a dead viper. Guinefort had saved his son.
Dismayed and horrified by the mistake, the knight and his wife decided to bury Guinefort in the most dignified way possible, filling in the well they had dug and where he had put it, covering it with stones and planting various trees and flowers around it, in such a way that it constituted an authentic sanctuary.
Aware of the incident, the locals began to go to the grave to honour the dog as a martyr, as they regarded him as a protector of childhood.
The villagers of that time used to pray to the dog saying “Saint Guinefort, protect us from idiots and evil snakes.” Saint Guinefort was awarded some miracles and his tomb became a pilgrimage site for centuries, from the Middle Ages until well into the 19th century, but it never had the approval or sympathy of the Catholic Church in Rome, because it was a dog and according to the Church, dogs do not have an immortal soul like people.
The story was collected and narrated in the year 1250 by Stephen of Bourbon, a Dominican inquisitor who enjoyed a certain prestige in his time.