Genesis 25:3

And Jokshan begat Sheva and Dedan; and the sons of Dedan were merchants, and negotiators, and chiefs of peoples.

ASSHURIM, AND LETUSHIM, AND LEUMMIM. Rashi comments: “These were the names of clan chieftains. But I cannot reconcile Onkelos’ translation with the language of the text.” as meaning “camps,” Letushim as “owners of tents who spread about in all directions,” and Leumim as “islands” or “sea-districts.” Upon this Rashi comments that he cannot make Onkelos’ translation fit in exactly with the words of the text. Ramban now proceeds to explain Onkelos’ understanding on the verse and also the reason why he was forced to make this translation.
It appears to me that Onkelos was of the opinion that Asshurim means camps, companies that travel the roads from city to city, just as it says, a company of Ishmaelites. Onkelos thus considered Asshurim as being derived from the expressions, To His steps (‘ba’ashuro’) hath my foot held fast; His steps (‘ashurav’) do not slide. And Onkelos’ opinion of Letushim is that they are the ones who dwell in tents that are scattered over the face of the earth, resting today in one place and tomorrow in another, for the letter lamed and nun interchange in many places, [thus Letushim would be like Netushim (scattered ones)] just like lishchah and nishchah, [both of which mean “chamber”]; And on that day men were appointed over ‘haneshachoth’ (the chambers)., 12:44. From this root is derived the expression, a sword ‘netushah,’ which is the same as letushah (sharpened). And of the word Le’ummim Onkelos said ulenagvon, [which is Aramaic for the Hebrew word] iyim (islands).
Onkelos was stirred to this translation by the word hayu [and the children of Dedan ‘hayu’ (were) Asshurim, and Letushim, and Leummim], when it would have been proper to express it similarly to the verse, And Mitzraim begot Ludim, and Ananim, and Lehabim, and Naphtihim.
And in Bereshith Rabbah we find: “Rabbi Shmuel the son of Rav Nachman said that even though we translate these names and say, ‘Merchants, flaming ones, and heads of peoples,’ all of them were heads of peoples.” The matter is as I have explained. The translators rendered Asshurim as “merchants, those who walk the road.” From the word Letushim (sharp, shiny) they derived “men of wickedness,” their faces are faces of flame, burning as torches, from the roots: to sharpen ( liltosh ) his plowshare, and his coulter; He sharpened ( yiltosh ) his eyes upon me. But Rabbi Shmuel the son of Rav Nachman said that even though they are accustomed to thus translate these names, they are still only the proper names of the heads of people, there being no descriptive name among them at all. And such is the case.

ויקשן, The Torah mentions only some of the children of Keturah’s sons. In the case of Yokshon even a grandson of Keturah is mentioned. Altogether the Torah mentions by name 16 such sons and grandsons. We are hard-pressed to know the reason for the Torah’s selectivity in mentioning only part of the many grandsons, etc. Perhaps those whom Avraham or at least Keturah still knew before they died, are mentioned by name, whereas those who were born by his/her sons after he had died were not mentioned. Considering that Avraham had married Keturah when he was 140 years old or older, he lived for only another 35 years, it would have been unlikely that he saw grandchildren from Keturah, considering that we have no record of anyone marrying below the age of 29 in those days.

אשורים ולטושים ולאמים, we do not know why the names of these three sons appear with the plural ending ים at the end. Possibly, these sons knew why they had been named thus. It is interesting that מצרים, a son of Cham, also gave his sons names ending with the ים plural ending. According to Onkelos, the names mentioned here were not their real names, as he translates their names as למשריין, ולשכונין, ולנגון.