The Wolf in Sheep's Clothing is one of the fables of Aesop.
The wolf put on the fleece, and went off in search of a flock of sheep. It spied a flock of sheep just as the sun was setting, and approached the flock. Just as it was about to pounce on a lamb, a shepherd came by, looking for a sheep to slaughter for supper. Thinking the disguised wolf was a sheep, the shepherd quickly grabbed and killed the wolf.
An alternative version is, another wolf is sneaking around looking for a sheep for dinner, and pounces on the wolf in sheep's clothing, killing it and eating it for supper instead of a real sheep.
The intended lesson was, "Frauds and liars are always discovered, eventually, and pay for their actions accordingly." The moral is sometimes also told as "The evil doer often comes to harm through his own deceit".
The image of a wolf in sheep's clothing is common in many cultures, such as it being echoed in 15, which states, "Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves." (KJV).
The phrase was also the inspiration behind the Airwolf series from the 1980s.
The Big Bad Wolf used this disguise tactic in Disney'sThree Little Pigs in an attempt to fool Fifer and Fiddler Pigs.
"Wolf in Sheep's Clothing" by This Providence is another song