A Wolf in Sheep's Clothing

This one is straight out of the Bible, from Jesus's Sermon on the Mount in the Gospel of Matthew: "Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves." It's a warning about people, especially religious teachers, who present themselves as trustworthy while actually being harmful. The image reached English through early Bible translations, including John Wycliffe's 1382 version. A cluster of similar-sounding Aesop's-fables-style stories about wolves disguising themselves in sheepskins also exist, but none of them are genuinely ancient; the earliest can only be traced to around the 12th century AD, more than a thousand years after Aesop, and were falsely attributed to him later.