Black sheep
A ‘black sheep’ is a member of a group who does not fit in. It’s often used to talk about the member of a family who is different or who makes non-traditional choices. It comes from the idea that most sheep have white wool, so a black sheep is the odd one out.
‘My brothers are very successful but I'm the black sheep of the family.’

Wolf in sheep’s clothing
A ‘wolf in sheep’s clothing’ refers to someone who hides evil, or negative intent, under an act of kindness or friendship.
‘The new manager seems nice, but she is a wolf in a sheep’s clothing.’

Separate the sheep from the goats
This idiom means to choose people or things of high quality from a group of mixed quality. 
‘We have a lot of applications, it will take a while to separate the sheep from the goats.’