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Up until gen 6 fucked everything up, the lore was pretty consistent that Pikachu was only native to Viridian Forest and nowhere else.
In Hoenn it can only be found in the safari zone, implying it to be imported from elsewhere. The same can be said for Sinnoh where it's only found in Mr Backlot's garden where its also implied to be imported. In Johto and Unova, they couldn't be found at all.
