After the death of Oden, all of the regions rise up to take down Orochi, but they were defeated by the Beasts Pirates. Shimotsuki Ushimaru and Onimaru, the fox who stole shusui from Zoro, were a famous duo. There was orginally going to be a scene when Onimaru is stealing Shusui, he is shocked by the appearance of Zoro, who looks identical to young Ushimaru; but that just makes things complicated so it got cut, besides it might not even be brought up in the manga.
Anyway I'll answer your question, Ushimaru is not Zoro's dad. These three characters' name won't be revealed in the manga, so I include them here. Regarding the lineage of Ushimaru, I'm not sure whether I'll draw more or hide it, so let's end the discussion here. They really do look alike huh?