Being between betas, I can't check myself, but does anyone know where in La Noscea the "new dragon skeleton" is? Does it match up with where Cassiopeia Hallow was? Based off my memory, the new skeleton is also a lot bigger than the one from Cassiopeia Hallow? This leads me to think they are different serpents, though they could be similar or same species. If there are two different names (Thalaos/Perykos), an easy assumption could be these are the two serpents.

Size wise, the new skeleton seems to fit the Serpent from Limsa's opening, but yeah the fact it's in rock, is already a skeleton and bridges/stairs are built on it post Bahamut, I doubt it's anything that was alive in Version 1.