Nearphotison:
I don’t know if there was a historical figure analogous to Thorfinn in the Dragon Ball world. Guessing at the history of Earth in that world is kind of a crapshoot. However, I feel like there probably were, at the very least, Vikings of some sort, since many other historical cultures are referenced at different points in the series. At any rate, I thought it’d be cute if Gohan thought Vikings sounded cool in his history books, and that lead to this situation here.