1. The police have been looking for that criminal for months. He approaches people and pretends he is selling them valuable stocks that are really worthless. He’s a wolf in sheep’s clothing.
2. John is such a good-looking young man, women are attracted to him quickly. It’s easy to see why people who don’t know him think he is probably a wolf in sheep’s clothing, when he is really a gentleman.

The expression comes from Aesop’s fable of the wolf that, in order to get close to a flock of sheep it wants to eat, clothes itself in a sheepskin to avoid detection.