That's just fan art. I think Big Bull only wore a mask in Madworld and he looked male. But then again, everything was so low-res in Madworld that it was hard to tell what was going on with the black and white SD graphics.
Yeah I googled, its definitely fan art because there isn't any information about it. He is a male in madworld and apparently since he got so f'd up by jack he got turned into the cyborg big bull we have now.