Reading modern English is one thing, but understanding Shakespeare is an entirely 'different beast'. [=is an entirely different thing; is entirely different]
a story about a mythical/imaginary 'beast' [='creature'] with two heads
a conflict between man and 'beast' [=between human beings and animals]
(..) - Yeah. - But only one of us was listening. - No, I was listening. I mean, that's why I'm doing this, to prove that I can handle it. - I'm not becoming a dark beast, right? - But that's not the point. Most women would thank a guy for that, but you're treating me like a common criminal. (..)2004 TvShow: Charmed Title: I Dream of Phoebe Season: 6 Episode 15