The idea of a person turning into a dangerous animal has been part of folklore ever since folklore was invented. It's quite popular today, at least in fiction, since the transformation makes a great special effect. But there have been real people in various parts of the world who went down in history as practitioners or victims of lycanthropy. In other words, werewolves.
1589: Peter Stubbe
Peter Stubbe (also spelled Stumpp or Stumpf) was known as "the Werewolf of Bedburg." His name may have actually been Griswold; the name "Stumpp" could have come from the fact that he was missing his left hand. This only actual record of this curious case is in the form of a lurid pamphlet which was circulated in Germany as a sensational tabloid would be today. Only a later translation exists now. Some accounts peg Stubbe as a serial killer, who murdered and sometime ate his victims over a 25-year period. He was also accused of incest with his daughter, which produced a son whom he murdered. There is also speculation that Stubbe was completely railroaded for political purposes, or to calm those who were terrified of the demons that were killing the townspeople.
At any rate, as he was stretched on the rack, Stubbe told how he made a deal with the devil and received a magic belt that turned him into a werewolf when he wore it. He confessed to murder, incest, and cannibalism. Stubbe's execution on October 31st, 1589 in Bedburg, Germany was an exceptionally gruesome process. He was first lashed to a wheel, where the flesh was torn from his body, then his arms and legs were broken, then finally his head was chopped off. Then his body was burned. Stubbe's girlfriend (a distant relative) and daughter, both accused of incest, were also tortured and then burned alive. After the executions, a real wolf's body was hung in public, his head replaced with Stubbe's head as a warning to anyone else contemplating lycanthropy.
1598: Jacques Roulet
Jacques Roulet became known as "The Werewolf of Angers" or "The Werewolf of Caud" after two nearby towns in France. According to an 1865 account, a mutilated teenage boy was found in the woods with wolves spotted nearby. Then Roulet was found wounded and half-naked not far away. He was arrested and confessed to the murder. Roulet also said he was given a salve that transformed him into a wolf and had murdered and eaten others. Unlike other cases, there appears to be no clear record of Roulet having been tortured into making a confession, and he did not confess to making a deal with the devil. Roulet was sentenced to death for murder, lycanthropy, and cannibalism, but after an appeal he was judged as mentally ill or "feeble-minded" and instead committed to an insane asylum for two years.
1573: Gilles Garnier
In the town of Dole, France, a series of children went missing and were later found torn apart in the woods. During the autumn of 1572 (timeline accounts vary), townspeople were charged with finding the werewolf responsible. In November, a hunting group witnessed a wild animal attack on a child, and someone recognized that the beast had features that resembled the local hermit, Gilles Garnier. A week later, when another child disappeared, Garnier and his wife were arrested. Fifty witnesses testified against Garnier, and he was put on the rack. He confessed to hunting, killing, and eating children who ventured into the woods, and said he shared the meat with his wife. In January of 1573, Garnier was burned at the stake. Modern speculation is that Garnier was guilty of murder and cannibalism, as he found children easier to catch than wildlife. But the werewolf confession is attributed to either mental illness or torture.
1521: Pierre Burgot and Michel Verdun
The Werewolves of Poligny were three men accused of lycanthropy in France in 1521. Michel Verdun was arrested when he was found dripping blood (possibly from a wound), and under torture not only confessed to being a werewolf, but implicated Pierre Bourgot and Philibert Montot. Bourgot in turn confessed, and told a tale of making a deal with three mysterious men dressed in black to protect his sheep. Bourgot said he only found out later that the deal entailed renouncing God and his baptism. He said in the years that followed, Michael Verdun gave him an ointment that turned him into a wolf, and together they killed at least two children. It is not clear whether Philibert Montot ever confessed, but he was executed along with the other two accused werewolves.
1685: The Wolf of Ansbach
One notorious werewolf case involved an actual wolf. In 1685, the Principality of Ansbach was part of the Holy Roman Empire, and is now a district in Germany. A man-eating wolf preyed upon livestock and then moved on to eating people. The citizens thought they were being terrorized by a werewolf, who was their unnamed (and hated) former mayor in the guise of a wolf. A hunting party with dogs drove the wolf into a well, where it was killed. Still believing it was a werewolf, the citizens chopped off the wolf's nose, dressed it in a man's clothing, added a human mask, and hung the body from a pole. A rather spooky drawing recorded the hanging. The carcass was later installed in a local museum.
1044: Vseslav of Polotsk
Vselav was the ruler of Polotsk, a region that is now part of Belarus, from 1044 to 1101 C.E. History records him as a strong leader and warrior, but he also said to be a sorcerer. In Russian literature, he is called Vseslav the Sorcerer. Folk tales pegged him as a werewolf soon after his death, and this reputation was recorded in the 12th century poem The Tale of Igor's Campaign. In it, the prince was said to race from town to town as wolf.
1651: Hans the Werewolf
In the 17th century in Estonia, a series of witch trials and werewolf trials brought charges against dozens of people accused of supernatural crimes. An 18-year-old named Hans was convicted of both lycanthropy and witchcraft. He admitted to being a werewolf for two years, but denied making a pact with the devil. Hans told a story of being bitten by a man dressed in black who later turned out to be a werewolf. The court decided Hans must have made a satanic deal, which made him guilty of witchcraft as well. The teenager was, of course, put to death.
There are plenty of other cases of real people who were accused of turning into werewolves, but these stories are old and the documentation is incomplete, unclear, and many have been altered and added to over time. Modern cases are considered psychiatric problems. Many possible explanations for historical lycanthropy have been offered, from rabies to hallucinations induced by ergot poisoning. But the most reasonable explanation as we see from the vantage of the 21st century is that fear of the unknown led groups to find someone to blame for the horrors of wild animals that killed people.