Quoted By:
"Regional variants" (recolour Digimon) only exist due to the limitations of Digimon World.
The recolours (ModokiBetamon, RedVegiemon, Dokunemon etc.) are the ones you can't recruit or obtain, while their normal colours are the ones you can.
The obtainable Digimon are the ones from the first 4 V-Pets, with MegaSeadramon, Phoenixmon and HerculesKabuterimon and many non-recolour non-obtainables (Gotsumon, Tentomon etc.) being from the Pendulum series which mostly came out after the game had been released.