I mean it said that two gods made leviathan to fill the world with water. So with that is it not save to say that they made the other primal to give life to the world? Ifrit for fire Garuda for Wind. Ramuh to give lightning to the world and shiva to give ice to the world. Seeing how each era in the game brought something to the world i remember reading that they was a time that the world was full of Ice. There 6 elements in the world of FFXIV and so far the primal that we going to fight are part of the element they also been 6 era in our world.

Now with the Seventh Umbral Era this brings strong powerful primal and monsters to the world. Odin could be a primal of his own we also going to see king behemoth which may just be a powerful monster in the game.

The information of the gods making leviathan could just be a story told by the people of eorzea and it may not be true in that world. But if it is then i guess it make sense how we going to see titan,ramuh and leviathan some point in 2.0. Also it make sense on primal being made by the gods seeing how there 2 of each element they could have made Demigod which are the primal of eorzea.