I've seen this claim on a number of "useless fact" websites but I cant seem to find the original source for it, so Im coming here to settle it. Is this true?
The source for this is a treaty of jurisprudence written by Guy du Rousseaud de La Combe, a 18th century lawyer, Traité des matières criminelles suivant l'ordonnance du mois d'août 1670. It can be found in Part 1. Crimes & Punishments in general, Chapter 2. Crimes & Punishments in particular. Section 1. Of the Crime of Lust. Distinction 8. Crimes against nature. p. 55. Here it is in glorious Latin:
Tertia species omnium horrendissima, cum quis venerea exercet cum animantibus brutis ; Ex Levit. 20. interfici debet cum bruto : indignum enim esset & odiosum tale brutum subsistere & in conspectu hominum versari. In eo nefandissimo crimine cum bruto non requiritur consummatio, sufficit conatus actui proximus, vide Boërium loco cit. num. 3. & 4.
Quantum autem ad hujusce poenam criminis irrogandam, mos olim erat, ut prisci referunt criminalistae, tale brutum illico cum homine peccante igni esse concremandum, simul cum lite, ne ulla post patrati sceleris punitionem remanerent vestigia. At novo quaesitorum Tribunalis Supremi Senatus Parisiensis decreto 12. Octobre 1741, Sententia Senechalis Curiae Pictonum fuit confirmata, qua adolescens quidam Picto, qui venere cum vacca abusus fuerat, in honorariam muletam, igneque comburi vivum, damnaverat ; hancque animantem brutam esse mactandam, ejusque membra humo condere decreverat.
This was not witchcraft, but bestiality, and it's in Latin because of the subject matter (the rest of the text is in French).
So: a young man from region of Poitou (not Paris) had sex with a cow, and he was sentenced to be burned alive. The cow was slaughtered and its remains buried. (My Latin is rusty, so please Latinists correct me if I'm wrong). When a man was accused of bestiality, he (or she) was often put in a bag with their "partner" and they were burned alive together. This worked when the animal was a sheep, and it may have been more difficult with a cow.
Charles Berriat-Saint-Prix, a jurist, wrote in 1829 a lengthy article about animal trials, which is a legal analysis of such trials from a historical perspective, and he provides a list of 85 trials that took place in France (except for one in Switzerland and one in Canada) from 1120 (against mice) to the above trial of 1741. Berriat is often cited so I guess that his expurgated version of the cow trial made the story popular, as this was among the latest animal trials recorded in France.
Animal trials were definitely a thing in France and in Western Europe, with the bulk of them happening between the 14th and the 17th centuries. There were several kinds of trials: those involving domestic animals (usually pigs, and often infant-killing/eating pigs), judged for killing or hurting people; groups of wild animals, frequently "vermins" such as rats, insects, worms or slugs (more rarely wolves or wild boars), accused of destroying crops or herds (they were typically excommunicated by religious courts); bestiality trials, where both man and animals were accused; witchcraft trials (usually cats, in the 16-17th century). For those interested, a larger compilation of about 200 animal trials was established by Edward Payson Evans (1906).
As noted by historian Michel Pastoureau, the historiography of animal trials is "disappointing". It has been mostly treated as an amusing curiosa, and with the exception of excommunication trials, actual information is scarce. One of the most famous case is that the Sow of Falaise, found guilty of killing a newborn in 1386, and who was tried and executed in public. This story, mostly known through the receipt of payment sent to the executioner, had attained the status of "historical legend" through successive retelling and embellishments (Friedland, 2012). I'm not sure that there is more to the cow story of 1741, but I haven't search the literature on bestiality trials.
Cows were more likely to be executed for killing someone. This was what had happened in in Beaupré in 1499 (a "red-haired bull" had "furiously killed" the young boy in charge of keeping the herd), and in Leipzig in 1621, where a cow had killed a woman (Evans, 1906).
Sources