well from my logic the oldest sets of pokemon should be either from the deep sea origins (deep sea= dont evolve= old), the guys that are from the mountain areas that in a shape of a sea creature ( must have been related) or the "ancient" form of those guys evolving via ancient power ( they return to the form they used to be at ancient times), and the last is those guys that dont evolve but knows ancient power ( they always like that since ancient times)