I place OoX before FSA, and thus, Ganon is still the demon referred to in FSA.
I think it's very possible that the Trident was a creation of the Interlopers though. It was then acquired and given to OoX Ganon, who was a resurrected Demon [as FSA talks about].

I think it's possible that Ganon's soul is now harbored in the Trident, or alternatively, because it once belonged to him, the Demon Ganon seeks out the new owner of the Trident [FSA Ganondorf] and indwells him.

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Voted as Best Zelda Theorist in the Summer '08 Awards. Thank you to all who voted, and to all who inspired me along the way!