"Whoa Cunningham" or "Old Cunningham" rather than "Whoa, Goddamn" only makes more sense if you are not a teamster. When I am driving and the team won't stop when I tell them or signal with the lines, then "Whoa Goddamn" makes a lot of sense.

Cunningham would only make sense if that was one of the oxen's names, and I doubt that was what was meant with Whoa Goddamn.

Seems to me the bull wacker is also cursing the maker of the back band, part of the harness, as it seems to be failing.

When one ox does not work he threatens to replace him with another by suggesting his "pappa is going to cut that other little bull" and make him an ox (which of course is a castrated bull).

Twenty yoke of oxen is a huge team and very unlikely in the example Leadbelly gives. A 20 yoke would only make sense in pulling several freight wagons.