According to one theory, he was the eighth son but he refrained from calling himself 八郎 (Hachiro) that means the eighth son, to demonstrate his respect for his uncle MINAMOTO no Tametomo, famous for his heroic bravery, whose popular name (kemyo) was 鎮西八郎 (Chinzei Hachiro), but it is only a folk story.
一説には実は八男だったが武名を馳せた叔父源為朝が鎮西八郎という仮名_(通称)（けみょう）であったのに遠慮して「九郎」としたともいわれるが、伝説の域を出ない。