Up to 30 incidents have been reported in pastures across the country, including cut ears, eyes removed, genitals cut, sides cut and blood drained.
No meat is taken from the carcasses.
Some of the gruesome acts could have been ritualistic mutilations by an unknown cult, while there are other theories of a “scary” social media challenge, or copycat crimes.
In the majority of attacks, one ear – usually the correct one – was cut off, echoing matadors taking trophies from an arena.
The Minister of Agriculture, Julien Denormandie, declared that he “excluded nothing” from the investigation and that “all means are in place to put an end to this terror”.
He added: “The ears are cut off, the eyes removed, an animal is bleeding out. ”
The crime scene ranges from the mountainous Jura region in the east to the Atlantic coast in the west, and many are taking place this summer.
A police spokeswoman in Paris said: “We don’t understand the motivation. Is it a satanic rite, insurance fraud, macabre trophy hunt, or an internet challenge? We do not know. It’s very traumatic. “
There is speculation as to how such barbaric acts, some of a surgical nature, could be carried out without knowledge of equine anatomy or on a horse that could presumably flee.
Veterinarian Aude Giraudet, head of the equine division at the Alfort National Veterinary School, said: “A fearful horse in a pasture will not get caught.
“The horse who feels confident with people… he will come, will find it normal that you put a harness or a rope around his neck. I’m not sure you need a great deal of knowledge of horses. ”
She said an ear can be cut off while the horse is standing, but the animal should be prostrated for more gruesome mutilations.
A man clashed with two assailants in his animal shelter last Monday in a village in the Bourgogne-Franche-Comté region.
The couple fled in a vehicle.
Nicolas Demajean injured his arm during a fight with an intruder who had a knife, while the other cut the sides of two ponies, traumatized but recovering.
He told France 3 TV channel: “I used to be confident putting my horses out to pasture. Today, I’m scared in my guts. ”
The next day, a young pony was targeted in Saône-et-Loire. In another case, some organs of a horse were removed.
The president of the French Equestrian Federation offered to help the police investigate the cases.
“We are all afraid,” said Véronique Dupin, manager of an equestrian club in Yvelines west of Paris, asking that the exact location of the team not be identified.