OK, I think I know what happened. Generally, the basic word is "zwierzę". This option is the best one and definitely should be accepted. But sometimes there is some bug and the correct option is rejected.
Now, there is a word "zwierz" in Polish. It's not that neutral, and personally I wouldn't accept it. But this exercise is shared with "English for Polish speakers" course and they accept it. So it's also among the accepted answers for us.