Because Susano is one of the thousands of Kami worshipped by the Yanxians and Hingans. In the same way that the western beast tribes all worship their deities, the primals, which they believe to be the primal essences of creation. We've adopted the term "primal" and misuse it to mean "any summoned deity" similar to how the Garleans adopted and misuse the term "Eikon" to mean something similar: "false god." So the two terms aren't mutually exclusive. Susano is a Kami, but because he was actually brought from belief into reality makes him also a (airquotes) "primal" because we Eorzeans now associate a term once used to describe beast tribe beliefs, to now mean summoned gods, because the beast tribes', the primals, were the first to be successfully summoned in Eorzea.
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The Lord of the Revel Journal Wrote:...summoning forth from the aether the great kami Susano─a primal by another name.
Fernehalwes Wrote:I can mention why primals are referred to as primals, though. It is not because they are underdeveloped, younger deities, but because the beast tribes tend to believe that they were the ones who created them, the world, and everything. They are the primal forms of existence.
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Lorecast 9 Wrote:Gamer Escape: How did the summoning of Susano work? He kind of came out of nowhere, there! If I had to guess, I’d say the Magatama acted a bit like Zantetsuken, but that’s the best I’ve got.
Koji Fox: This one was in the forum questions, as well, but I can answer you guys here. Actually, there’s nothing new going on, at all. It’s the same way primals have always been summoned: strong belief, the power of the crystals—aether within the crystals consolidates to make a manifestation. Until now, the beast tribes have been like, “Okay, we need Ifrit NOW!†“We need Garuda NOW!†and the belief and aether in that area would result in a manifestation of that belief. With Susano, there was no intention, but everything else just fell into place. There was a ritual in a sense—intense belief, aether latent in the treasures, and even though it wasn’t intended, it just sort of happened. It wasn’t a mistake, just happenstance.