So there were only 150 known Pokémon, then suddenly 100 more were discovered in the west, then more were found in Hoenn and Sinnoh and so-on.

If these 600+ other Pokémon were discovered over the course of a few years, how is it that there are ancient legends and other history revolving around these 'newly discovered' Pokémon that without a doubt occurred prior to Professor Oak's '150' statement?