Ascended Fanon: The fan theory that Giovanni and Team Rocket had some connection to Mewtwo (whether it be in its creation like in the anime and Pokemon Adventures manga, or as what they wanted to capture when they gained control of Silph Co. and its Master Ball) has been given credibility with Giovanni possessing Mewtwo in Ultra Sun and Ultra Moon.
Gengar is the favorite Pokémon of Ken Sugimori, one of the franchise's main artists.
Series creator Satoshi Tajiri's favorite Pokémon are Poliwag and Poliwhirl.
Psyduck for composer and future game director Junichi Masuda.
Development Hell: Development of Red and Green was notoriously rough, due to Game Freak being a somewhat small firm at the time with little experience in Game Boy development and absolutely no experience with making a game as unique as Pokémon. This is why the Generation I games are so poorly-coded compared to their successors, as well as why a lot of gaps exist in the game's balancing.
No Export for You: The original games were not released in South Korea due to the ban on most Japanese cultural products still being in place until 1998.
Remade for the Export: The international releases were based on the Japanese Blue version, which fixed a number of glitches and had a host of graphical improvements, particularly to the battle sprites.
Sleeper Hit: Nintendo of Japan thought the games wouldn't do well while releasing Japanese Red and Green and wrote them off as a loss. They didn't top the sales charts, but kept selling steady in a market where 80% of sales are made in the first two weeks. Game Freak made a few tweaks, released an updated version (Japanese Blue), and Nintendo got their cash cow.
There's a creepypasta about a hacked version known as Pokémon Black (not to be confused with Pokémon Black and White), found on a black-painted cartridge. The player is given a Pokémon known as 'Ghost' instead of one of the starters, and it cannot be put into the PC. It attacks opponents with Curse, and if you return to an area after beating a trainer, it's indicated they died as there's a tombstone present. After beating the game, the player character's Pokémon are all gone and the sprite is swapped for an old man sprite. Going back to Pallet Town results in the character being attacked by the spirits of the Pokemon they killed on their way to beating the game, with the character using Struggle until they die. Its existence is questioned, but a ROM like this does exist online. Article here
Missingno. (or Missing Number) has had a number of rumors around it, some said catching it would damage your cartridge others say that it would erase your data, and Nintendo itself would tell players encountering it would be harmful to their games to dissuade players from pursuing it, but in reality all it does is give you 128 copies of your sixth item and mess up your Hall Of Fame data. Other glitch pokemon, on the other hand, can cause more severe effects to your file when encountered and caught.
There's a truck sprite that only appears once in the game in the S.S. Anne harbor. The only way to see it is to go way out of your way to avoid getting the Cut HM from the ship captain, as that permanently closes off the area once you leave without exploiting glitches, and then returning when you can travel on water, as the truck can only be reached in the harbor when you have Surf. People believed that it had to have been put there for a reason, leading to hundreds of rumors about this thing. As it turns out, it's just a graphical easter egg with no actual benefits associated with finding it. As a reference to these rumors, in FireRed and LeafGreen, checking under the truck gives you a Lava Cookie.
There are various rumors about how to catch Mew, many of which involve moving the truck in Vermilion City in some convoluted fashion. While it is actually possible to catch by abusing glitches, none of them involve the methods perpetuated by the popular rumors.
Speaking of Vermilion City, the S.S. Anne was said to return there after the player beat the Elite Four so many times. There is actually a way to re-enter the S.S. Anne through a glitch, but it does not involve beating the Elite Four in any way.
Some old cheat lists claim that your Mons will receive a "revenge boost" to their Attack when their other stats are decreased. This one is actually partly true; while nobody intentionally programmed that feature into the game, one of the game's myriad bugs causes badge boosts to be reapplied every time any of their stats gets modified. That said, any move that modifies a stat will force a proper recalculation of that stat, wiping away these glitch boosts. It's not revenge, it's just a glitch.
There is some claims that Gyarados originally had a Water/Dragon typing but was changed to Water/Flying after Gamefreak realized how broken the typing would be (as in Gen 1 with there existing no real Dragon moves besides the Fixed Damage Attack Dragon Rage and being many Generations before the Fairy type's introduction, this typing would have absolutely no type weaknesses). There exists no evidence however that Gyarados was ever meant to be part Dragon type, and even when it got a mega evolution in Gen 6 that gave it a new secondary typing, it still wasn't made part Dragon type.
There were prominent rumors back in the day about rare hidden pokemon like the infamous "Pikablu" and "Mewthree", with extremely convoluted methods to finding them. Of course Mew is the only hidden pokemon in the game, and none of these other rumored hidden pokemon existed.
The unreachable tall grass seen outside the fences in Pallet Town and the seeming path beyond Bill's house were a hotbed source for rumors, often connecting with the Mew and other hidden pokemon rumors, with people claiming that were where you found them. But there exists no additional area that Bill grants you access to and the Pallet Town tall grass is merely just aesthetic, with the game even crashing if you use a walk-through-walls cheat or glitch to walk into the grass.
What Could Have Been: At one point, there was to be a special music track for link trading, that would be played on both Game Boy systems as a duet. The data for it still exists in an incomplete form in the final game; this is the well-known Dummied Out music track which was originally speculated to be town music. More information, as well as a restored version you can listen to, is available at The Cutting Room Floor.
Trivia for Yellow:
Keep Circulating the Tapes: An even more severe case than the originals, since at least those were remade for the GBA. Rescued along with the originals on the Virtual Console.
Recursive Adaptation: It's a game based on The Anime of the Game, and in addition to some Canon Immigrants, there are some touch-ups on sprites and Pokémon used in Gym battles: Brock and Misty wear their respective outfits from the anime (in the original games, Brock was shirtless and Misty wore a bikini), and Lt. Surge only uses Raichu in battle, which is what he also did in the anime.note He used 3 Pokemon in Red and Blue, and the remakes and later games follow this instead.
Trivia for FireRed and LeafGreen
Ascended Fanon: Originally, "Leaf" was not considered an official name for the female trainer in the GBA remakes.note She is given this name as a placeholder in the game's data, but this data also refers to Blue as "Terry" and therefore should be taken with a grain of salt. However, it was a popular enough Fan Nickname that her Kotobukiya ArtFx J figurine, revealed in 2018, officially gives her the name リーフ (translated as "Leaf"). However, Pokémon Let's Go, Pikachu! and Let's Go, Eevee! overrode this by naming her "Green", but then Pokémon Masters released afterward would go back to calling her Leaf. If Leaf and Green are meant to be separate characters is unknown, but in either case Leaf is now a recognized official name for at least one version of the character.
Distanced from Current Events: The model of the space shuttle in the Pewter City Museum is no longer referred to as the "Columbia" anymore, most likely in response to the destruction of that particular space shuttle in the year prior to the remakes' release.
Fandom Nod: As a reference to the famous truck rumor, the pier that the truck sits on contains a Lava Cookie, which is normally unavailable until you reach the Sevii Islands.
Fan Nickname: The name "Leaf" for the female Player Character was initially never stated to be her actual name, and is based on one of the placeholder names on the ROM for FireRed and LeafGreen. This name would be officially adapted as her canon name for some versions of her character.
Urban Legend of Zelda: It's said that if you have a fake copy of the game, or if you play the game in a very outdated emulator (preferably, one that was released before the game itself), the Seagallop ferryman in Vermillion City will say "By the way: If you like this game, buy it or die." This message is actually only in some very early ROM dumps of FireRed, and in reality, was the work of the pirates who managed to leak the game early.