A common idiom is 'wolf in sheep's clothing', which refers to someone or something that seems friendly, but might be out to harm you. This idiom comes from a fable by Aesop, whose moral is 'seek to do harm and harm will come to you'.
The boy who cried wolf
This tale concerns a shepherd boy who repeatedly tricks nearby villagers into thinking a wolf is attacking his flock. ... The moral of the story is that liars will not be rewarded; even if they tell the truth, no one believes them.
