Pikachu's Origin?

What do you think the origin story of Ash's Pikachu is? It's never been revealed, has it? It had to have come from somewhere before coming into Oak's possession, after all. Discuss, if you wish!

I always thought it was just a random, wild Pikachu Oak had caught.
Either to study it, or offer to an older trainer possibly, or both.

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?

My thought is that Pikachu belonged to a trainer that gave up on it because of the uncooperativeness it showed in the beginning. The uncooperativeness may have been specifically because of Ash, but I think it's possible Pikachu was like that with its previous trainer(s) before and that's why Oak had it available to give Ash after the three starters were taken, figuring that Ash wasn't going to get too far anyway and it would be right back there soon enough.