Nearly all villains Luffy has face or the villains of arcs have been mostly based off animals. While not all are animals the majority are.
Arlong-Saw tooth Shark
Smoker-Not being counted
Crocodile- Is named Crocodile
Rob Lucci-Has Leopard...
This is a crazy theory that I saw on a one piece blog (can't remember which one) like three or four months ago. Because Scratchmen Apoo apparantly became Kaido's underling, this theory becomes even more likely.
It all started before the timeskip. Scratchmen Apoo and the On Air Pirates were...