It's all alchemy, guys.
Solgaleo is steel type and a lion because the green lion eating the sun represents plants growning with sun's light but also represents sulphuric acid "devouring" all metals except the gold, so it's life and metal's purity, while Lunala is a skeleton which represents corrosion and putrescence of metals and death. At same time, the golden sun and the silver moon represent the male and female sex, just like Solgaleo and Lunala are said to be Cosmog's male and female final evolutions.
Cosmog itself represents part of the creation of the philosopher stone as was wrote into the Emerald Tablet, a gaseous-like creature born of its father and its mother (Solgaleo and Lunala) and raised on Earth ("The father of it is the sun, the mother the moon. The wind bore it in the womb. Its nurse is the earth, the mother of all perfection."), Cosmoem, "the cocoon of stars" is the philosopher stone itself and the final evolutions also represent two phases of the transmutation, "albedo" and "nigedo".
Necrozma is the cube, the prism that represents body in which sun and moon (Cosmog's line) are poured completing the transmutation into the perfect being, which is usually represent by a dragon.
The red color of Cosmog's line in its shiny form is another reference to the philosopher stone, which is fully linked to the red color, the last phase of transmutation is even called "rubedo" for that (albedo = whiteness, nigredo = blackness, citrinitas = yellowness and rubedo = redness).
I guess the part of Necrozma going gold regardless you use Solgaleo or Lunala represents the citrinitas, which is the part of transmute into pure gold, not needing the reflective "lunar or soul light" anymore)
Oh also dusk and dawn forms are clearly based on a manticore and the crab grabbing the moon (the cancer symbol, which represents Digestion like leo, the lion represents Dissolution)
