snake in the grass

A treacherous person, as in Ben secretly applied for the same job as his best friend; no one knew he was such a snake in the grass. This metaphor for treachery, alluding to a poisonous snake concealed in tall grass, was used in 37 b.c. by the Roman poet Virgil ( latet anguis in herba). It was first recorded in English in 1696 as the title of a book by Charles Leslie.

The noun has one meaning:

Meaning #1: a deceitful or treacherous person
  Synonym: snake