This card is "Mythical" because it has ever been professionally certified or sold with receipt in a public auction, and Wizards of the Coast has not confirmed or refuted the production error or test print.

There are multiple sources mentioning this version of Base set Raichu was used to test the Prerelease stamp before it was used in production on the Clefable card in the Jungle set.

The story from Hobbylark cites the exsistance as an error, where the Prerelease stamp was unintentionally printed on Raichu instead of Clefable.

They allegedly destroyed nearly every copy of the Raichu promo, leaving only a few dozen left that were said to be handed out to employees of Wizards of the Coast.