The Pikachu must've been one of those residing in the Viridian Forest. After sent out to face the world all by himself by his parents, he came wandering to the area near Pallet Town, wary and very cautious of all the humans as his parents must have told him. Acc. to RBY, I beleive Oak caught that Pikachu while trying to protect some pokemonless kid who was about to receive a pokemon on that day, and was about to head over to Route 1 after checking out each and every nook and cranny in the entire Pallet in search of the man himself. That kid may have been one of the three who became the early birds, and were able to catch the worms.. i mean the Kanto starters. Then Ash would come running inside, and would find out, to his horror, that the proffessor was out of the starters, and then, Oak, considering the fact that postponing Ash's journey for the next couple of months, until the hatching of the new set of charmander, squirtle and bulbasaur eggs, wouldn't be the best news the kid had ever received, decided to get rid of, and hand him over the newly caught Pikachu, the little yellow mouse being not at all happy about its capture. And the rest is history..
So, how was my theory?
