It's because Green Bull is his nickname. Ushimaru had a Green Bull on his cloak and his name has bull (Ushi) in it.

I don't think he's Ushimaru anymore but he could be his son and thus Zoros father potentially.

Somehow this may parallel Helmeppo and Morgan too somehow potentially or maybe not lol.
Phew... I see. Let's see what Oda makes of these potential hints/links in the story. (I have become a bit careful with theories and hype around Zoro, because I kept being disappointed by the past couple of chapters and the lack of what could have been a great arc for zoro fans honestly.)

PS: Also welcome back! I am glad you decided to stay around! :)
 
ZKGB
Holyshit, Greenbull will neg diff the rest of the SH and Luffy won't be in the condition to fight anymore, but Zolo will overcome twice the pain and wake up again to save Wano from Admiral bastard :suresure:
 
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