Is it at all possible that there may be two primals per element the same way there are two gods per element? There's still a lot of mystery about the rest of Hydaelyn. Perhaps the beast tribes around the world used to know of both, but favored one so much for so long that the other was forgotten over the eras. This leaves an option of at least hearing about other primals from far away lands in future patches and maybe, just maybe, meeting them.

If, however, there are absolutely only six primals in all of Hydaelyn, then perhaps they can change how they look to best impress whatever group summoned them. This means that the Ala Mhigan youths who went to summon Rhalgr with some ritual they knew nothing about may, in fact, have been summoning Ifrit who would then have taken on a more humanoid shape because that's what they expected. Or he may have fallen on them as he does with the Amalj'aa. We'll never know.