TLDR, at least one potential Base Set Prerelease Raichu’s have surfaced over the years. There is a debate if the card is legitimate/fake, and how it was produced. It’s unlikely there will ever be a definitive answer. Mentioning Prerelease Raichu usually leads to an endless debate about its legitimacy, its value, etc. For more info, search ‘prerelease Raichu’, interesting rabbit hole to go down
With promos included, across multiple languages, that may not hold true though. We are talking some highly exclusive cards and the fact that we’re trying to buy all of them gives sellers leverage to drive prices higher