The latest in Cortambert, a village in the Saone-et-Loire region, occurred overnight on Friday.
The horse's right ear was cut off, one of its eyes was gouged out and its genitals had been cut away. "This is absolutely barbaric," a local official said.
A Paris police spokeswoman said there had been about 10 similar cases across France since the start of the year, most in the north and many taking place in secluded meadows.
She said there were also cases between 2014 and 2016 and that similar killings and mutilations of horses had been reported in the past in Belgium and Germany.
But the high number in France was unprecedented over such a short period.
"We do not understand the motivation. Is it a satanic rite, insurance fraud, some macabre trophy hunt or an internet challenge? We don't know. It is very traumatising," she said.
She said all types of horses had fallen victim, as well as one donkey, and that mutilation of an ear was a common factor. No meat had been taken from the carcases.
National police are assisting local investigations of cases, and France has sent alerts to other European countries' forces, she said.
R.C.