The death of a wolf in the French countryside in 1767 put an end to a town's fear that it was a victim of witchcraft, of a lycanthropic sorcerer, or of the depredations of a demon come out of Hell itself. But the three years of vicious attacks which spawned these theories remain a source of speculation and fear to this day. It's also one of the few examples of cryptozoology in the most literal sense - an animal whose true nature remains hidden to scholars.
The story of the "Beast of Gévaudan" begins in 1764, when a woman in that town saw what she described as a large wolf approach her in her fields; the horns of her cattle kept it at bay, and she was unharmed. But while this attack was the first, it would not be the last, and many were fatal - one estimate attributes some 113 deaths to the Beast in all. Its hunting methodology was consistent: it would use its enormous height, putting its forepaws on the shoulders of its victims and biting their heads.
Stories had it that the Beast was immune to bullets (it was shot multiple times with no apparent effect), which only lent further credence to supernatural explanations. Illustrations change, over the time in which it was active, from showing a very large wolf (as described by most early witnesses) to something more demoniac in appearance. Meanwhile a military captain, Jean-Baptiste Duhamel, took charge of the effort to hunt it down, organizing the townsfolk into search parties, but to no avail.
By order of King Louis XV, a reward was announced in the spring of 1765 for the capture or death of the Beast, and by the fall it was dead. The specimen was mounted and sent to the court, where naturalists affirmed that it was a monstrously large wolf, weighing in at 143 pounds. But two years later, the attacks resumed - there was, it seemed, a second Beast. (One modern theory attributes the attacks to an entire pack of wolves, of which at least two members were sufficiently large to inspire the eyewitness accounts.) This second beast was killed by a hunter named Jean Chastel, and shortly after a smaller (but still quite large) wolf was killed nearby. These deaths put a stop to the attacks at last.
Chastel is often reported to have made his kill with a silver bullet, cast on the hypothesis that the Beast was a werewolf. No primary source confirms this detail.
Today, there is still a great deal of debate over the nature of the Beast. Some have proposed that it was a wolf-dog hybrid, which reportedly sometimes grow larger than either parent; the dog is supposed to have been some sort of mastiff, which would explain the stocky build and great weight of the Beast. Others suggest a pack of unusually large wolves, with the first Beast killed as the largest. Ranging further afield, the hypothesis that the Beast was a pair of panthers or of striped hyenas, most likely escaped from a menagerie (possibly even one belonging to a relative of Jean Chastel), or that its attacks were the work of a serial killer who bred a large dog or wolf-dog for this precise purpose. A politicized version of this theory features in the French film Le Pacte des Loups, which shows the Beast as a hyena-like creature dressed in armor to give it an even more terrifying appearance. Given that other prominent attacks by less monstrous wolves are especially well-documented in mid-18th-century France, however, it seems most probable that some one of more mundane explanations is the reality - but as the taxidermed remains have been long since lost, we will likely never know the full truth.