According to Bretonian Folklore, the Mourioche is a malicious demon of bestial nature, able, it would seem, to transform himself into any animal shape he chooses.

Mourioche has always haunted Brittany, spreading his brand of cruel humor along the coastlines of Côtes-d’Armor and around Jugon-les-Lacs. He is a water-horse, and a shapeshifter; there is no end to the forms he has assumed, and he loves using his powers in creative ways. Mourioche is usually seen in the form of a yearling colt, pig, cow, or sheep, often with a pair of muscular arms. But in general appearance he is like a year-old foal.

Mourioche comes out at night, and preys on nocturnal travelers. Sometimes he is a horse standing by the side of the road, waiting for riders. His spine stretches as more and more people get on, then he gallops right into the lake, his laugh echoing in the darkness. At other times he wrestles passers-by, grappling with his brawny arms and throwing his victims into muddy ditches. He will jump onto men’s back and force them to carry him until they drop of exhaustion. He will follow people along the road, changing shape every time they turn to look at him, and making a sound like tearing canvas.

Drawn-out sadistic pranks are Mourioche’s favorite form of entertainment. A farmer of Saint-Cast once found Mourioche in the form of an abandoned ewe, and took him home to his barn. The next day, when he went to check on his new sheep, he found a cow; the day after, it had become a horse. On the fourth night, it was a sheep again, who laughed and said “Why do you check on me every morning? You’re weird!” It was then that the farmer saw that all his animals had been slaughtered. He reached for his shotgun, but Mourioche took off, destroying half the barn and abducting the farmer’s three children (who were never seen again). Mourioche is not without mercy, though, and he left behind a golden necklace.

Mourioche is not without his faults, however, and is baffled by anyone who doesn’t fear him. One man nonchalantly carried Mourioche all the way back home, and the shapeshifter fled when he called his wife. Another time Mourioche took a tailor on his back, who threatened to cut his ears off with his scissors. The tailor was returned to dry land very quickly.

Mourioche is especially dangerous to children, and Breton babies are often chided when noisy or mischievous with the words: "Be good, now, the Mourioche is coming!" Of one who appears to have received a shock, also, it is said: "He has seen the Mourioche".

Nobody knows for sure where Mourioche came from. Some say that he (for lack of a better pronoun) was once a Breton man or a woman, versed in the dark arts, who sold their soul for a magical ointment. Other accounts make him a simple werewolf without control of his actions. Dubois whimsically claims he was once the court jester of an undersea kingdom, and was banished for bad behavior. There are even claims that he is the Devil himself.