well, nimaiyo is the creator of the zanpakuto. since he himself is a shinigami, it only makes sense that he would have had the first zanpakuto. also, they call yama's blade ancient, never the first. actually we haven't seen many fire type zanpakuto at all.
ichibei doesn't even call bankai, bankai. which gives the impression that he is used to calling it "true strike" or whatever the chapter called it, or simply referring to it as the evolved form of the zanpakuto. he made a point to say kids now a days call it bankai. meaning that he later decided to name it this. nothing in what was said gives the impression that there were many bankai before he learned his. just that he didn't name it until much later.
When it comes to first there are three possibilities:
- Yamamoto was on higher authority than Oetsu and for that reason got first zanpakuto.
- Yamamoto didn't got first zanpakuto, but still his zanpakuto is one of oldest.
- Oetsu's zanpakuto and Yamamoto's zanpakuto is not same. Since Oetsu didn't really explained his zanpakuto. Japanese translator say that when he say "Sayabusa" it's not name of his zanpakuto like Ryujin Jakka or Kyoka Suigetsu, but that is different type of Asauchi.
There are Yama's, HInamori's, Isshin's and Love's zanpakuto. 4 Fire type zanapkuto, there is no type with more zanpakuto.
He refer to it as envolved form, because it was ancient name for it, before he renamed it to bankai. Since he is the guy who name things, he would give name momentally bankai. Just for the fact that it first had fake name, tell you that there were bankais before his. Beceuse if there weren't, name would never be envolved form.
