The closer Dialga and Palkia come to Arceus' likeness, the more of its power they obtain. So I'm going to return to the old lore of USUM.

Necrozma, Solgaleo and Lunala represent the Body, Soul and Spirit, the three requirements for creating the philosopher's stone and attaining God - Arceus. That's why, although not quadrupedal, Ultra Necrozma begins to take a form similar to Arceus.

This was a Gen/Region where Silvally, a mock-up of Arceus, was a thing. So don't tell me they didn't have Arceus in mind when designing Pokemon for this region.