Genesis 36:12

And Timna was the concubine of Eliphaz bar Esau, and she bare to Eliphaz Amalek. He is Eliphaz the companion of Job. These are the sons of Adah wife of Esau.

AND TIMNA WAS CONCUBINE TO ELIPHAZ ESAU’s SON. Because Scripture was not particular to tell us the names of the mothers of all the others, our Rabbis have interpreted that this was to tell us of the esteem in which Abraham our father was held, i.e., how eager people were to attach themselves to his descendants. This Timna was a descendant of chieftains, as it is said, And Lotan’s sister was Timna, [and Lotan was one of the chieftains of Se’ir]. She said to Eliphaz, “If I am unworthy to become your wife, would that I might become your concubine,” as Rashi has written.
It is possible that the five sons of Eliphaz, mentioned in the preceding verse, were generally known as his children since he had begotten them from his wives. But Amalek, [born of Timna, the concubine of Eliphaz], was not known among his brothers, [who were the recognized children of Eliphaz], and he might have been included among Esau’s children because he was his descendant. Therefore, Scripture found it necessary to say that his mother so-and-so, to whom Amalek was known to have belonged, bore him to Eliphaz, but he is not listed among the descendants of Esau and did not dwell with them on mount Se’ir. Only the sons of the mistresses, and not the son of a concubine, are called Esau’s seed, since the son of the handmaid will not be heir with his sons, in keeping with the practice of his father’s father.
Now concerning the descendants of Esau, we have been commanded not to abhor them or take their land., 2:5. This refers to all his known sons who dwell in Se’ir, as they are called Edomites by his name, but the son of the concubine is not part of the descendants of Esau, and he did not inherit together with them in their land, and in fact with respect to him we have been commanded to the contrary, i.e., to abhor him and blot out his name., 25:19.
Now Rashi wrote further: “In the book of Chronicles Scripture enumerates Timna among the children of Eliphaz! This implies that he lived with Se’ir the Horite’s wife and from this union Timna was born. When she grew up she became his concubine. And this is why Scripture says, And Lotan’s sister was Timna, [since Lotan’s father was Se’ir the Horite]. And the reason why Scripture does not enumerate her among Se’ir’s children is that she was Lotan’s sister maternally but not paternally.”
But I do not agree with this since in the book Chronicles, it should have said, “and Timna his daughter.” Why should Scripture …

ותמנע היתה פילגש, I have found in a Midrash on Psalms (שוחר טוב) that the prefix ו before the word תמנע suggests that this word may be read as part of what follows or as part of what preceded it. In fact, in Chronicles I 1,36 בני אליפז, תימן ואומר צפי וגעתם קנז ותמנע ועמלק it is understood as belonging to verse 11 in our chapter.

ותמנע היתה פלגש, Eliphaz was one of the conquerors of that land and Timna a sister of the אלופים of the original inhabitants of the region fell to him as a conquest and became his concubine.

ותמנע, it is possible that the Timna mentioned here was not a sister of Lotan as opined in the Midrash (Bereshit Rabbah 82,14) where we were told that this woman’s name and husband is mentioned in order to further illustrate the tremendous esteem in which Avraham had been held internationally at the time. This woman, who was the sister of an Aluf, i.e. a well known dignitary, a man of great prominence, said that seeing she did not have the good fortune to marry a direct descendant of Avraha, she was willing to settle for the lower rank of being a concubine to a son of Esau in order somehow to become a member of this outstanding family.