There hath indeed been an old opinion of such like things; For by the Greeks they were called λυκανθρωποι which signifieth men-wolves. But to tell you simply my opinion in this, if any such thing hath been, I take it to have proceeded but of a natural superabundance of Melancholy, which as we read, that it hath made some think themselves Pitchers, and some horses, and some one kind of beast or other: So suppose I that it hath so viciat [damaged, tampered with] the imagination and memory of some, as per lucida interualla, it hath so highly occupied them, that they have thought themselves very Wolves indeed at these times: and so have counterfeited their actions in going on their hands and feet, preassing [attacking] to devour women and barnes [children], fighting and snatching with all the town dogs, and in using such like other brutish actions, and so to become beasts by a strong apprehension, as Nebuchadnezzar was seven years: but as to their having and hiding of their hard and schellie sloughes [scaly skins, like snakeskin], I take that to be but eiked [fabricated?], by uncertain report, the author of all lies.
— Daemonologie of King James, 1597 (I modernized the spelling)
Medical descriptions of clinical lycanthropy — the delusion that one has turned into a wolf or other animal, along with corresponding animal-like behavior — date back to classical antiquity. As far back as the second century AD, the Greek physician Marcellus of Side described lycanthropy sufferers as melancholics who “roam out at night and mimic the ways of the wolves or dogs and mostly loiter by the grave monuments until daybreak.” Marcellus considered them harmless, both to themselves and others.
Arab physicians expanded on the Greek concept of lykanthropoi, splitting it into two. The harmless kind they called qutrub, after a type of jinn or ghoul who haunted graveyards and ate corpses. They also described another condition, mania lupina, a more violent malady whose sufferers behaved wildly and wolfishly, and often could only be restrained with shackles.
Stories of humans transformed into various animals, including wolves, appear in Greek and Middle Eastern mythologies, but the werewolf as we understand it today probably had its origins in Nordic mythology. From Scandinavia the motif spread, showing up widely in the rest of Europe by about the 14th century, not just as folktale or legend, but as superstition. In Western Europe werewolves were associated with witchcraft; in Eastern Europe, with vampires (according to Baring-Gould the Serbian term vlkoslak denotes a creature both werewolf and vampire).
Given the association with sorcery, werewolf trials did occur during Europe’s witch hunt period (roughly the 15th through 18th centuries), but the view of lycanthropy as a mental illness also overlapped the superstition, as shown in the quote from Daemonologie that began this post — and James was pretty credulous about the existence of demons and witches. Even during the witch trials, the courts sometimes recognized self-confessed werewolves as mentally ill, rather than demonic beings .