Conspiracy Theory
Female dwarves disguise themselves as male while traveling
Fíli and Kíli introduce themselves as sons of Dís instead of using a patronym
The throne of Erebor skips over the previous King’s surviving sibling and goes straight to a vaguely distant cousin instead
Fíli is treated like the heir apparent, despite being Thorin’s nephew instead of a son (& therefore heir presumptive, i.e. assumed instead of clearly defined)
Dís is the only dwarf woman listed in geneology, because of the “valor of her sons”, and their father is never listed
She’s also the only female dwarf named in canon
Conclusion: Thorin is actually Dís, thanks for your time
Dàin becomes king after Thorin, Fíli, and Kíli all die in battle, but if Fíli is Thorin’s heir that implies that the throne can be inherited through the female line. Thorin is the eldest of his siblings so there’s no way to really tell whether it’s male-preference or absolute primogeniture, but either way it means that Dís cannot be completely excluded from the line of succession. Under Salic or Semi-Salic law Fíli would be excluded from the line of succession and Dàin would be Thorin’s heir, and we know this isn’t the case. So either Dís renounced her right to the throne in favor of her son (unusual but not unheard of, though the only example I remember is from the post-medieval period) or she was already dead.
Furthermore Thorin’s name (along with most others in the company) ends with the grammatically feminine single N ending (more on that here by someone who understands it) meaning that it’s entirely possible that Thorin was a female dwarf. With this additional evidence it becomes even more unlikely for the throne to skip Dís entirely. BUT, since Dís’ name is obviously feminine and Thorin’s appears completely unisex except in writing, it could be that she took up “Thorin” is an alias, as it’s an established fact that dwarf women pretend to be male while traveling.
[insert “always sunny” conspiracy cork board screenshot here]
I also appreciate an alternative ending, where it’s Thorin that died, and Dis basically took over his identity when dealing with non-dwarves.
Thorin could die at various points - maybe he doesn’t make it through the Battle of Azanulbizar, maybe he dies later due to fighting or injuries or misadventure. The middle brother is already dead, so Dis becomes either the official heir or ruler in her own right, depending on whether Thrain is still alive at this point. She uses her own name among dwarves, but Dis adopts Thorin’s name as a use-name among other races. It’s a convenient alias - Thorin is already established as both male and the eldest child of the house; Dis using his identity gives her more political weight among humans and elves.
So when it comes time for the Quest, it’s Dis we see, using Thorin’s name. She brings both of her sons along - they all have to keep up the story that they’re nephews and not sons, but the relationship is still close enough that they don’t have to pretend not to be close.
