Curious bit of language history: the phrase 'wolf in sheep's clothing' is a mash-up of ancient moral warning and folk storytelling that stuck in everyday speech.
The earliest clear textual echo lives in 'The King James Bible' — Matthew 7:15 warns, 'Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.' That biblical metaphor popularized the image of something dangerous hiding under a harmless guise, and it fed into sermons, proverbs, and later literature across Europe. Around the same historical ballpark, a fable often attributed to 'Aesop's Fables' and preserved by Phaedrus tells of a wolf who disguises itself in a sheepskin to slip past the flock. When its deception is discovered, the wolf is killed, which cements the moral: deceit rarely succeeds without consequence.
Over centuries these two streams — scriptural caution and folk fable — braided together, and by medieval times the idea was a common trope in moral tales and courtroom speeches. In modern usage the phrase shows up everywhere from political commentary to pop culture, because the image is instantly evocative. I love how a single compact phrase can carry centuries of warning and storytelling; it's one of those language fossils that still bites with relevance.