Who were the Refa'im?
Rashi #1: The Refa'im were synonymous with the land over which Og Melech ha'Bashan ruled. 1
P'mimim mi'Shulchan ha'G'ra: Refer to 15:20:2:1.
Why does the Torah omir the Chivi from the list?
P'ninim mi'Shulchan ha'Gra: The Refa'im are synonymous with the Chivi. 1
See P'ninim mi'Shulchan ha'G'ra, who equates three of the names here with three of the missing names of the sons of Cana'an in No'ach, in chapter 10, and also why Tzifon and Sini are omitted here.
QUESTIONS ON RASHI
Mizrachi: Eretz Refa'im was the land inhabited by the Chivi (whose name is missing on this list). Og was their king, despite being an Emori, for he was a giant like they were.
Gur Aryeh: The Emori lived on both sides of the Yarden River. Those on the eastern side were known as Refa'im, and Og was their king. Those on the western side were known simply as Emori. 1
Rashi: Refer to Devarim, 2:20:2:1.