Trying to follow עשו's three-dimensional family tree is hard enough. Following the offspring of שעיר החורי which follow the listing of עשו's is made equally difficult by the recurrence of the name דישן. Sei'ir named his fifth child דִשׁוֹן. Then, in a stunt mimicked by the Saskatchewan Roughriders and Ottawa Rough Riders of the Canadian Football League thousands of years later, he named his seventh child דִישָׁן. Both appear in ל"ו:כ"א and later on in ל. Fair enough. We let the CFL get away with it, we can let שעיר get away with it as well. However, the confusion grows when the פסוקים list the grandchildren of שעיר. In פסוק כ"ה, his fourth child, ענה, decides that there are simply not enough דישן's in the family and names his son דִּשֹׁן as well. But when the Torah lists the children of שעיר's fifth child (פסוק כ"ו), formerly referred to as דִשׁוֹן, he is referred to there as דִישָׁן. Two פסוקים later, his brother is called דִישָׁן as well. This is more confusing than the CFL (The CFL had eight teams; שעיר only had seven children.)
The גר"א suggests that the spelling of the names is dependent on the placement of the word in the פסוק. If the word appears at a principal stop, that is, either the end of the פסוק or the אתנחתא that signifies the principal pause in the פסוק, then it is written דִישָׁן. When it is not, it is written דִשׁוֹן. When we are introduced to them, the fifth son is at the beginning of the pasuk and thus is written דִשׁוֹן. The seventh is on the אתנחתא in the פסוק and thus, is written דִישָׁן. When listing their children, both appear in the middle of the פסוק and therefore, both were written דִישָׁן. The proof to this theory is that in דברי הימים א א:ל"ח both are written דִשׁוֹן because both are not at the end or middle of the פסוק.
The only difficulty with this is that in דִישָׁן ,פסוק כ"ח appears on a טפחא note, not אתנחתא, an apparent violation of the גר"א's rule. To reconcile this problem, we must turn to אמת ליעקב in פרשת בהעלתך where he writes that when a פסוק does not contain an אתנחתא, the טפחא may take on the roll of the primary pause as in במדבר ט:ב and י"ג:ח. Here, too, the טפחא, in the absence of an אתנחתא, takes on the roll of the primary pause and turns דִשׁוֹן into דִישָׁן.
In the end, שעיר's fifth and seventh sons were both named דִשׁוֹן but that name changed to דִישָׁן based on the placement in the pasuk just as many other words have a vowel changed to a קמץ when at the end or middle of the פסוק. The גר"א, however, did not forget about ענה's son in פסוק כ"ה. He was also named דִשׁוֹן. But he was a different kind of a דִשׁוֹן. He was a דִשׁוֹן that didn't change despite the placement in the pasuk. So, here, and in דברי הימים א א:מ"א, even though his name falls on an אתנחתא, it is written דִשׁוֹן. That's one confusing family.