It would easily be survival of the fittest. Be a Pokemon trainer or die from a maniac's Pokemon or a rampaging wild Pokemon. Humans would not be able to survive long enough to develop Pokeballs or other capture methods.
Fast food chains would find themselves in an awkward moral position over the prospect of Miltank slaughter... vegetarians over the prospect of eating Oddishs... (Although they may count as meat? Anyone know the physiological make-up of an oddish??)
Oddish is likely either an animal sharing a symbiotic relationship with a plant, or is a plant with similar adaptations to animals, such as a brain and muscles. Either way, though, vegetarians would likely be opposed to eating Oddish because it has all the qualities of an animal: it's self-aware, unique, and responds to stimuli in a similar way to animals.
The worst part: it actually does go through sometimes
Check Grovyle's Japanese PDex entry. Just lovely.lol That's so true. Little kids would get kidnapped by Drifloons and Banettes, peoples souls would be stolen by Lampents and Shedinjas, Gengars would be stealing peoples lives, and dead people would turn into Yamasks. How lovely.