I would love to say that this was my theory, but I'm just relaying something I read on an anonymous forum. RE: Elezenos

It's Estinien.

The reasoning here is that he was the one who destroyed Nidhogg's eyes, releasing the last bit of aether they had back into the world. Zenos used the power of the eyes and his own Resonance to control - and literally become - a primal. We know from the Sahagin priest that it's possible for a soul with the echo to "hop" from one body to the next, and given the power of Zenos and his Resonance combined with the timing of everything, Estinien makes the most sense from a purely FF14 standpoint.

Next we have the historical allusions, cause we all know SE and Yoshi love to slip in references to previous FF games. Estinien is basically Kain from FF4 in a lot of ways. His iconic armor, the DRG AF1 set, is a very faithful recreation of Kain's armor from FF4. Kain was "mind controlled" twice over the course of FF4 while Estinien has been "mind controlled" (or possessed if you will) once, during Heavensward. This would mark the second time, and given Estiniens past was filled with loss and suffering, it makes sense Zenos would be especially drawn to it. Plus there's the side benefit of Estinien being a battle-hardened veteran of hundreds of battles, so Zenos can more or less pick right up where he left off as far as physical combat ability goes.

In sum, it's a combination of larger "meta plot" reference and allusion coupled with the time and place of Zenos' death along with whoever was in proximity at the time.