They are the so-called koshi koden books and, because the original manuscripts themselves were lost in a fire or were not open to the public, they are deemed as 'gisho' (apocryphal documents) or 'gishi' (apocryphal history).
いわゆる古史古伝の書物であり、写本自体が焼失もしくは非公開であるため、「偽書」「偽史」とされている。