If you haven't been watching the Dev Blog like a hawk since it first went online, and all the shame on your house if that's the case (:p): Shiva in FFXIV. The colors you see there are not the colors of her render. Those screens are an in-progress ZBrush project marked with contrasting colors for clearer definition and prevention of the eye strain that comes with looking at a colorless gray slab for weeks to months on end. Color Concept Art Here
Unknown! It could be that they have a singular creator who modeled them how they pleased. It could be that their visual appearance is based on the belief of their summoners somehow. Who knows! I imagine it's linked to why the Egi doesn't look like the primal. Garuda-Egi looks far more like the Empress of Birds than the Lady of the Vortex. All my theories on the matter are speculative, and if you hang around here long enough you'll learn that while I hit a few nails on the head on occasion, my theories have a tendency to make a lot of sense and then get proven completely wrong more often than partially right, lol.
