There actually was a Beast of Gévaudan (La Bête du Gévaudan) which was a real wolf-like creature that prowled the Auvergne and South Dordogne regions of France during the years 1764 to 1767, killing about 100 people, often in bizarre circumstances.
Apparently in the belief that no one outside France has any sense of history, the translators writing subtitles omitted a historical reference in old d'Apcher's memoir. The subtitles read, "The Revolution has swept the land," but in French he says, "The Revolution has become the Terror" (this may have been changed in some DVD versions).
Universal Pictures paid $2 million for the rights to distribute this movie in the United States, and this movie went on grossing $11.2 million in limited theatrical release in the United States, making it the second-highest-grossing French-language movie in the United States since 1980 (this movie also did brisk video and DVD sales in the United States).
The trivia items below may give away important plot points.
There really was a "Beast of Gevaudan." Historical records of the "Beast's" attacks, which served as inspiration for the movie, allege that it was a wolf-like creature though it was often described with fanciful or demonic elements to its character; including an unusual color, a supposed resistance to bullets, and cloven hooves. Various explanations for the beast's identity and behavior have been proposed over the years, ranging from a mastiff-wolf hybrid trained to wear boar-hide armor to a member of an exotic now extinct Asian Hyena species (in the film the beast is a lion in spiked metal armor). However no evidence has ever been found to suggest anything other than an unusually large and aggressive wolf was the culprit. Officially, as in the film, "The Beast" was a large grey wolf who was shot and killed by the King's hunter in September of 1765. However, further attacks plagued the region until June of 1767 when a local hunter shot and killed a wolf which, upon examination, was found to have human flesh in its stomach, and is now believed to have been the true "Beast of Gevaudan."
A deleted scene in the DVD revealed that Monica Bellucci's character, Sylvia, was in fact the only person who knew the whole truth to both sides of the story regarding the beast. She secretly watched several members of the Brotherhood while they were visiting the bordello, and she manipulated Fronsac into finding and killing the beast. Her character is believed to have worked for the Vatican.