A wolf in sheep’s clothing

Generated image of a wolf in sheep's clothing
The idiom a wolf in sheep’s clothing describes someone who seems kind or harmless but is actually dangerous or dishonest. It warns people not to trust appearances.
This idiom comes from an old story where a wolf dresses up as a sheep to sneak into the flock and eat the sheep.

We thought he was here to help, but he was really a wolf in sheep’s clothing.