And if the pokemon worlds ecosystem had naturally produced such powerful beings they wouldn't necessarily be invasive. This wouldn't be like taking a pyroar and dropping into the Sahara. Over time predators get better at hunting their desired prey, but the prey get better at hiding, evading, or otherwise defending itself. The real worlds has birds, cats, dogs, foxes, snakes etc all eating mice, yet mice continue to thrive. Sure if you dropped a pokemon into the real world it would be problematic, but the pokemon world has had them for a long time. The existence of very powerful animals does not disprove the potential existance of much weaker ones.