It's true that Pichu didn't exist until Gen II, but wasn't Gen II already around by the first episode of the anime? Ho-oh was in it, after all. Seeing as Pikachu was feral, and hated pretty much everything, that gives the impression that it was newly-caught, and wasn't used to being around people. I don't think Oak evolved it from a Pichu, but it has to have evolved from a Pichu.
I mean, even thought Pichu still wasn't discovered, Pikachu must have evolved from it. Just because things haven't been discovered, it doesn't mean they don't exist.