Yeah it can be confusing so I hope we get it clarified soon.
I posited above that the Death Devil might in fact be the horsemen's mother, which would make sense, as the Death Devil should be more powerful.
How Fujimoto decides what Primal Devils are I have no idea lol. When googling primal fears these are the results that show up the most often
No idea how accurate this is tho.
With some stretching you could argue Darkness falls under extinction maybe. Or mutilation seeing as people were losing limbs when fighting it. Death would be in Extinction in that case. Idk lol. Hopefully we get this cleared up soon.