I also support Finno-Ugric mythologies as a plural term, because there is no just one mythology or system of myths, but different and  diverse Finno-Ugric mythologies. Some kind of an analogy for this is how we name the groups of languages; we do not speak of (common) Finno-Ugric language, but Finno-Ugric languages (plural). Baltic-Finnic mythology is still acceptable as a singular term, because Baltic-Finnic mythologies share very much with each other. Kalevala, for example, is collected from speakers of different baltic Finnic languages. But there could not be an opus of Fenno-Ugric or Fenno-Volgaic mythology; it would just be a mess of unrelated material and languages that are not mutually intelligible.  puhu