Origin of the Primals

[Brightamethyst]

Ifrit - We were teased with the concept of Ifrita created by Lahabrea pre-Sundering but we don't know how, or even if it's related to Amalj'aa mythology.

Garuda - The leader of the Iksalions back in the 4th astral era. Probably became the goddess of legend as centuries passed and stories changed.

Moggle Mog - Likely another actual historical figure who evolved into myth and legend over time.

Shiva - Hraesvelgr's lover, become patron saint of Ishgardian heretics. There was quite of lot of plot about it.

Bismarck - Almost certainly based on the fey of the same name and design. Whether the original still exists on the Source or not is an unknown.

Thordan - Again, this was explained pretty clearly in the MSQ.

Bahamut/Phoenix - Bahamut is explained in the MSQ and Phoenix is likely just based on the firebird rebirth legend.

Everything else - Unknown. We can assume based on how myths and legends evolve in the real world but there's no way to know anything for sure until the game specifies.

Bonus: Enkidu - Inspired by... Enkidu.

[Enkidoh]

Moggle Mog - Likely another actual historical figure who evolved into myth and legend over time.

He was, he was the founding chieftain of Moghome in the Churning Mists - the references in his story to "using a golden rope so that moogles could escape to the surface" is a cultural meme of the transporter in Mourn that allows travel between Sohm Al and the surface in Dravania. A moogle quest in Moghome during HW reveals this.

Bismarck - Almost certainly based on the fey of the same name and design. Whether the original still exists on the Source or not is an unknown.

Bismarck is an odd case as there are several legends involving him on the Source - as I recall it was a giant whale spoken of in pirate legends in Limsa Lominsa (and thus also gave rise to the name of the resturant run by the CUL Guild in Limsa as "The Bismarck"), but the version most known is the vanu vanu's stories of "the White" in the Sea of Clouds, a giant whale that was born albino and thus the other whales mocked him for his white skin, which the gods took pity on and gave him the power of flight, allowing him to float high in the Sea of Clouds, which is the version the Primal version is a bastardization of, becoming a mindless monster that simply flies around and devours islands.

Bahamut/Phoenix - Bahamut is explained in the MSQ and Phoenix is likely just based on the firebird rebirth legend.

Phoenix appears to have a prototype like Ifrit back in Amaurot - one of the Tales From the Shadows stories on the Lodestone mentions about an accidental creation by someone in the Akademia Aynder 12000 years ago who created an "immortal firebird" that somehow became imbued with a soul that was self destructive and kept trying to kill itself, only to regenerate, locking itself into a agonizing never-ending cycle of death and rebirth, (Emet, as Hades, was brought in to finally return the soul to the Lifestream and thus destroy the creature). Phoenix's name was never mentioned, but by all descriptions it was Phoenix. Which raises the question, how did old man Louisoix come up with the same concept? I think we will never know.

[YianKutku]

Phoenix's name was never mentioned, but by all descriptions it was Phoenix. Which raises the question, how did old man Louisoix come up with the same concept? I think we will never know.

This might be explainable by the hypothesis that the legend of a fiery bird that has power over rebirth is something that appears and spreads through many cultures and beliefs, hence Louisoix thinking about rebirth in the context of Phoenix, and also Suzaku being mistaken for a phoenix-like deity over in the Far East.

If we stretch the speculation a bit, there might be the possibility that all of it can be traced to that short story Amaurotine accidental creation and the racial memories thereof, but I also think it doesn't really need that common link. Sometimes similar concepts in myth can develop in parallel, and then resemble each other even more due to people from different cultures meeting each other and going "oh yeah, that sounds just like this thing we have", then going back and spreading that version. This could even be on purpose, for evangelizing, which happened all the time in RL all over the world.