As we know from Legends, true form Arceus is a ball of light that cannot be comprehended sitting beyond space and time with powerful creative energy...

In multiple dex descriptions across generations it is referred to as emerging from an "egg".

Terapagos true form (or evolution), could be that "egg". What other purpose would a giant sphere on your stomach have? It can't be armor because it's face and back is exposed. If it's protecting it's stomach then it's quite excessive...

Maybe Terapagos and Arceus form a creation energy. Arceus beams of light / type / infinity energy bounces off Terapagos refractive hexagons like a Dyson Sphere charging up before hatching into creation.