I've a actually just looked up 'beast' in a dictionary and the first translation it gave me was 'zwierzę'. But... no, that's just too much here. 'Zwierzę' is a very neutral and common term and 'beast' just doesn't reflect that. Maybe it works in some collocations, but I wouldn't accept it here.
Zwierz is a similar, albeit literary word, which fits your suggestion much better.