How did Japanese players first get Mew?

Mew was not meant to be found. Programmer Shigeki Morimoto added it to Red and Green in the final two weeks of development, after debug tools were removed and a sliver of space opened up, without telling Nintendo. In Morimoto's own account, the team made Mew in two weeks as a prank that was never supposed to become anything more.

CoroCoro Comic turned the secret into a phenomenon. The magazine's May 1996 issue, on sale April 15, announced a Legendary Pokémon Offer: twenty winners, announced in the July issue, would send in their cartridges and have Mew written onto them, generated by Morimoto himself with Trainer IDs 00001 through 00020. Around 78,000 readers entered. Twenty Mew, distributed by mail, against the whole of Japan's playground demand.

The scarcity built the myth. Players who had never seen one traded stories of a 151st Pokémon, the rumors fed Red and Green's word of mouth, and the games' long sales tail became legend in the industry: a hit that grew after launch instead of fading.

Mew's official distribution widened later, but its first form is the purest object in early Pokémon: not a cartridge, not a card, just data twenty children won from a magazine.