That's artwork of Proto-Ultima (which is of course, directly referencing FFVI's Ultima/Atma Weapon). If you've ever caught a glimpse of him flying around during the super FATE weather in Azys Lla, that's what he looks like. While Ultima Weapon is a machine, it appears Proto-Ultima is some sort of chimerical creation like many of the things on Azys Lla. Whether it is some sort of suped up Reptoid or not is anyone's guess, but I feel that is far too simple an explanation for something that seems like it will be pretty important in the coming plotlines.