I called it ( Final boss of endwalker)

[Garnix]

The benchmark have already told us who would be the first trial boss, and he is Zenos.

You mean the same type of benchmark than the one that teased us Yojimbo? You know, that same Yojimbo which ended up being a dungeon boss in 4.0.
IMO, the various terminus beasts that we see in the benchmark could be the add phase during the Anima trial, and Zenos could be a dungeon boss...again...or a single player instance, again...