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 Pokémon 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.
Professor Oak is programmed into the game as a trainer and has a full team. It consists of five Pokémon that are even higher leveled than Champion Blue's team (capping out with a level 70 Gyarados), and one of his Pokémon is a fully-evolved starter, implying he would have used whichever one was left over from the beginning of the game. Whether he was originally intended to be the True Final Boss or some sort of Optional Boss is unknown. You can trigger a battle with him by exploiting a glitch or using a cheat device, though he has no dialogue and beating him gets you nothing more than EXP and money.
There's another type in the data called the Bird-type, presumably a earlier version of Flying. The glitch Pokémon Missingno. and 'M have this typing, but it doesn't have any weaknesses or resistances. Interestingly, a lot of NPCs refer to Flying as "Bird", which is even kept in the remakes.
Dragon-type moves deal super-effective damage against Dragon-types, however, this behavior never actually occurs in Red and Blue, because the only Dragon-type move that exists at this point is Dragon Rage, a Fixed Damage Attack that always deals 40 damage and ignores any type effectiveness.
Another accidental example with the attack Kinesis. It does exist in the game, but no Pokémon is capable of learning it, you'll only see it through usage of Metronome. Yellow makes it the Secret Art of Kadabra and Alakazam.
An unused Trainer class called Chief ("Silph's chief" in Japanese) exists, and can be found through glitches, implying the manager of Silph Co. was meant to be battled somehow. Unlike Professor Oak, however, he has no defined sprite or party; fighting him uses a Scientist sprite since that's the next Trainer class in the game's data.
When healing at a Pokémon Center, the game remembers where you are so you'll return there when using the Escape Rope item or Teleport move. However, there's a check in this routine that specifically excludes the three "rest houses" in the Safari Zone, which of course never happens because those buildings have no healing functionality in the final game.
Fan Nickname: The "Gen 1 Miss", which refers to the 1/256 chance of all supposedly 100%-accurate moves to miss. In all later generations, 100% accuracy means 100%.
Killer App: Red and Blue singlehandedly reinvigorated sales for the aging Game Boy, which ended up getting an upgraded version one month after the North American release of the games.
Late Export for You: The games reached America in 1998, two years after the original release, and in Europe a year later. The delay was due to the significant amount of work that went into the localization; the code was revamped based on the later Blue version, every Pokémon was renamed and trademarked, and thne all of the text displays were reprogrammed. Since it was in close proximity with the release of the Game Boy Color, this had the side effect of making the games look incredibly dated compared to its contemporaries, like The Legend of Zelda: Link's Awakening DX and Wario Land II.
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.
Release Date Change: The games were completed in October 1995 and set for release in December, but for whatever reason, they were delayed by two months to February 1996. This is why the games bear a copyright date of "1995" in both their original versions and foreign re-releases.
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.
Same Content, Different Rating: When the game was originally released in Europe in 1999, it received a 3+ rating. However, due to PEGI's changed terms over the years, the game earned a 12+ rating in its first Virtual Console release because of gambling elements in the Game Corner. Now, under today's standards, any game with simulated gambling would automatically earn an 18+ in Europe.
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 Pokémon 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
A rumor has always been around that the Lavender Town music had to be subtly changed when localizing the game, because Japanese players reacted badly to hidden undertones. Depending on the rumor, the reaction ranged from hearing loss to inexplicable nausea to committing suicide. All of these can be proven false by the fact that there were never any changes to the Lavender Town music between releases in Generation I.
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 Pokémon, 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.
Surprisingly, this does not apply to an exploitation of how the game engine processes data will allow you to trigger an encounter with any Pokémon (or trainer class, but that might not end well for you) of your choosing...including Mew.
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. Amusingly enough, four Generations later the games introduced the Defiant ability, which does just that.
There are some claims that Gyarados originally had a Water/Dragon typing but was changed to Water/Flying after Game Freak 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 Pokémon like the infamous "Pikablu" and "Mewthree", with extremely convoluted methods to finding them. Of course Mew is the only hidden Pokémon in the game, and none of these other rumored hidden Pokémon 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 Pokémon 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.
Dummied Out: The Old Man Glitch in RBY, which was the optimal way to quickly encounter a MissingNo., was patched in Yellow. It's still possible to find a MissingNo. in Yellow but it requires a lot more work and most likely will crash the game.
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 three Pokémon in Red and Blue, and the remakes and later games follow this instead.
Ascended Fan Nickname: 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"). Pokémon Masters also refers to the character as Leaf.
Distanced from Current Events: The model of the space shuttle in the Pewter City Museum is no longer referred to as the "Columbia" anymore in non-Japanese versions of the game, most likely in response to the destruction of that particular space shuttle in the year prior to the remakes' release.
FireRed and LeafGreen introduced item sprites, and even included Hoenn-exclusive key items even though it is impossible to obtain them normally. This feature is carried over to Emerald onwards even though they don't exist in Ruby and Sapphire.
An item sprite for Bug-type TMs exists, despite them not existing in Generation III. This was likely done to prevent the game from glitching or crashing if one was hacked in.
Fandom Nod: As a reference to the famous "Mew under the truck" rumor, the pier that the truck sits on now contains a Lava Cookie, which is normally unavailable until you reach the Sevii Islands.
Urban Legend of Zelda: It's said that if you have a fake copy of the game, or 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.
or using a cheat device, though he has no dialogue and beating him gets you nothing more than EXP and money.