copycat - a person who copies someone else, an imitator (ape)
My classmate always copies my ideas and tells that they're hers.
dark horse - someone with a surprising ability or skill no one was aware of (unexpected winner)
In fifth grade I was the dark horse in English class.
monkey business - silly or dishonest behavior.
When we are waiting for our teacher for example, and we are smashing everything in the classrom, the teacher walks in and tells us to stop this monkey business.
white elephant - an expensive but yet useless possession (more trouble than its worth)
The huge painting in my room is the white elephant of our house.
crocodile tears - unsincere tears
wolf in sheep's clothing - a crook, a deceiver
