Ara(荒) in Japanese can mean rough, rude or wild (my wild justice theory could be true, fingers crossed), while Maki(牧) can mean shepherd, pasture or feed. So I think his name may allude to his volatile nature, to be both tranquil like nature but also wild like a bull.
Before I forgot, I gotta admit to @rerere and @Steven that you both are right about Zoro's bounty. Now that Kidd and Law got 3 billion each, Zoro having 1.620 billion is no longer impossible. Congrats!
Heck, even 1.720 billion is longer impossible for him.