Él es un lobo con piel de cordero.

English Translation

He is a wolf in sheep’s clothing.

Unless this is a well known phrase, should it be oveja?

This is a well-known phrase and incredibly mild as far as idiom translations go considering its the same animal! It’s a good exercise to try to find idioms in the language you’re learning, compared to ones you’re familiar with in your native language. A lot of times these vary from country to country or region to region. I think one of the sentences on clozemaster is Estoy en el quinto pino