>>57929150 (OP)Because in APL, we learn that two clans worship “The Almighty Sinnoh” as a deity.
However, one group believes that The Almighty Sinnoh is a master of time (and therefore Dialga), while the other group believes that The Almighty Sinnoh is a master of space (and therefore Palkia).
At the end of the game, both clans discover that The Almighty Sinnoh is neither Dialga nor Palkia, but rather Arceus. This confusion is probably due to the fact that the origin forms resemble Arceus.
This statue probably symbolises the union of the two clans and their beliefs. Highlighting both the entity that governs time and that which governs space.
It was also a way for the DS to have a statue that referred to both Dialga and Palkia, without necessarily redesigning it.
So it's not a real Pokémon that ever existed, which is why this statue is never referred to as its own thing and never encountered.
And because it was built after APL, when the clans unified, you don't see such a monument in Hisui.