there are a lot of treads on what animal Franky is (personally I think a rooster) but what's Brooke?
heres what all the other mugiwara resemble..
Luffy=monkey
Zoro=shark
Nami=cat
Usopp=armadillo
Sanji=duck
Chopper=reindeer
Robin=robin (my guess. unconfirmed by Oda)
My guess is that Brooke is a CRICKET. Because of his musical talent and ability to jump around and surround his opponents with discieving sounds.