The tractate Avot consists of five chapters. At some point, a sixth chapter was "added". This sixth chapter is an adaptation from the eighth (or sixth, depending on edition) chapter of the 'minor' tractate Kallah Rabbati, a collection of Baraitot outside of the Mishnah with Talmudic discussions. It also sourced in Tanna D'vei Eliyahu Zuta, chapter 17 (subject of another discussion).
Who, if anyone, instituted this collection of Baraitot into association with tractate Avot? It is mentioned elsewhere on this site that the custom of reciting a chapter of Avot on each Shabbat of Sefirat HaOmer was related to the adding of the sixth chapter, so as to have what to say on the sixth Shabbat. I'd love to see a source for that as well. Did whoever added this chapter intend for it to become formally part of Avot, or was it simply some additional reading for his congregants?
Note: It seems clear that its' inclusion in Avot goes at least as far back as the Geonic period, but I'm looking for something more specific.