What does "who dwelt in the land" mean?
Rashi #1: They lived there originally until Esav came [and captured the land from them].
Rashi #2: They were knowledgeable in the ecology of the land - which area was good for planting olive-trees, and which for vines. And they were able to determine this by tasting the earth. 1
See Ba'al ha'Turim.
Why do we need to know the lineage of Se'ir ha'Chori's family?
Ramban #1 (citing Rashi): Due to Timna, to teach us the esteem in which Avraham Avinu's family was held (Refer to 36:12:1:1).
Ramban #2 (citing Ibn Ezra): To draw a clear distinction between Se'ir and Esav - seeing as Yisrael will later be warned against despising the Bnei Esav and against taking their land.
Seforno: The Torah is listing here the names of the mighty warriors of that time, to teach us that by the will of Hashem, Esav nevertheless succeeded in destroying them and taking over their land. 1
Peninim mi'Shulchan ha'Gra: All of these names are places. Since Esav took them, Yisrael are forbidden to take them. 2
Ohr ha'Chayim: This teaches what Hashem gave to Esav in the merit of his ancestors, even though he was a Rasha. We expound (Shabbos 85a) "Chori" - they became Bnei Chorin from their property (Esav took it).
Malbim (to Divrei ha'Yamim I 1:38): Just like there are 70 Bnei Yaakov corresponding to the 70 nations, so there are 70 chiefs from Avraham's family - 13 from Yishmael and his sons, 13 from Bnei Keturah and their sons, 16 from Esav and his sons, there were 26 chiefs from Se'ir, and 26 chiefs from Esav stood in place of them; 3 and Amon and Mo'av.
As the Torah states in Devarim 2:22.
Surely, the Isur to take Esav's land depends on what he owns at the time; Sichon's conquest of Amon and Mo'av permitted them to Yisrael! The Torah teaches us to assume that the owner of these lands is from Esav, unless we know that later, someone else conquered them. (PF)
What is his source that each chief was replaced by one chief? The Torah lists below (36:40-43) 11 chiefs of Esav who ruled [over everything that Esav took from the Chori]! And why does he not count the 11? And even if one chief replaced each chief, perhaps some were among the 16 listed! (PF)
Who were Bnei Se'ir ha'Chori?
Ramban: Hachori was the name of the founding-father of the nation, after whom the nation was called. Se'ir was the name of the country, so-called from the moment Esav (who was "k'Aderes Se'ar" (25:25) from birth) arrived there. 1
Ha'amek Davar: Se'ir ha'Chori was from Chivi ben Kena'an, like it says above, "Bas Tziv'on ha'Chivi" (36:2). The Chori nation already existed in the days of Avraham - "ha'Chori b'Hareram Se'ir" (14:6).
