Content Leak: The source code for the original games was leaked in July 2020, allowing several development builds of the game to be reconstructed. The content revealed included earlier designs for Gym Leaders and several species of Pokémon.
The Shellos line has a set of unused back sprites in Diamond and Pearl. The unused Shellos sprite's design is mostly identical to the West Sea form aside from having the East Sea form's back flaps, while the unused Gastrodon sprite is brown like the West Sea form but has several bumpy protrusions on its back.
Arceus was apparently planned to be obtainable via an event involving an item called the Azure Flute, which would allow the player to enter an area above Spear Pillar known as the Hall of Origin and encounter Arceus. The item was never released, allegedly because the developers considered the use of the Azure Flute too obscure to figure out, and Arceus was instead made available by direct digital distribution.
The Generation IV games have a ???-type Arceus in their coding, with a slate blue coloration, to match the ???-type existing for the move Curse. There's no way to access the forme without hacking in a corresponding Plate, but it was likely included to prevent the games from crashing in that scenario. The form is no longer present from Black and White onward due to the ???-type's removal.
The Sis and Bro trainer class from the Gen III games were going to appear (gender-swapped) in Generation IV, but ended up going unused.
A Mr. Mime pre-evolution and evolutions for Lickitung, Tangela, and a Grass-type Eeveelution existed way back in Pocket Monsters 2, the early incarnation of Pokémon Gold and Silver, before they were cut from the final product. These games, however, revive them as Mime Jr., Lickilicky, Tangrowth, and Leafeon.
Canalave and Snowpoint City were both intended for Pocket Monsters 2 (as places known as New Type and North/Blue Forest respectively) but were cut. They're brought back here, slightly modified.
Shellos and Gastrodon had been designed for Ruby and Sapphire, but were cut due to time constraints. They were used here.
Trend Killer: The success of these games essentially caused the collapse of the Antipokémon community. Though most of the sites didn't even last beyond the first generation, the stubborn few that lingered had been prematurely gloating that Ruby and Sapphire would be the end, only for the Popularity Polynomial that came with Diamond and Pearl to prove the franchise wasn't going anywhere and throw a giant egg in their faces. Not a single Antipokémon website remains active today.
Trolling Creator: The developers programmed an NPC who will trade you a Haunter in exchange for a Medicham. It's common knowledge that Haunter evolves into Gengar upon being traded, and said Pokémon is a fan favourite both in terms of design and competitively. However, the Haunter that is given to you is holding an Everstone, an item that prevents it from evolving. To rub salt in the wound, talking to the NPC afterwards causes her to giggle and confess she knew full well that the player would feel disappointed.
Urban Legend of Zelda: Rumors persisted prior to the games' release that the starters would follow a new elemental dynamic (Fighting/Dark/Psychic instead of Grass/Fire/Water). note Funnily enough, the Grass, Water, and Fire starters of Pokémon X and Y actually have Fighting, Dark, and Psychic as their subtypes, respectively.
Spiritomb was originally going to be called Phantomb.
Buildings were supposed to use the same 3D perspective (complete with 3D props such as tables) as the overworld instead of the forced 2D perspective.
A beta sprite leak revealed most Pokémon had beta gender differences, most of which were later scrapped. Some of this follows real-life logic; most or all of the male horned Pokémon have bigger horns than the females, but some are a bit stranger, like many female Pokémon having smaller ears, which makes no sense as this type of sexual dimorphism has never occurred in real-life. The decision to scrap all of these may have to due with all these sprites needing to be animated, and there are plenty of differences that would have just made the females needlessly uglier or less desirable to players. Some specific examples:
Some female Pokémon, like all of the female Eeveelutions, have stunted ears. Beta female Umbreon's sort of look like they've been cropped at the tip like some dog breeds' ears are. An interesting one is that female Pikachu had floppy ears in addition to the heart-shaped tail. The heart tail was kept for the final game, but not the ears.
A leak in 2020 revealed that the beta designs for Giratina and Arceus, while most likely placeholders, still looked very different from their final counterparts, with Giratina being a six-legged chimera-like being with feathery wings/crest and a two-tentacled mouth — far more mammalian in appearance than the reptilian appearance of the final — and Arceus being a quadrupedal, amorphous, shadowy entity. Darkrai's beta design was also more humanoid and corporeal than the final version.
Beta sprites◊ of the Sinnoh Gym Leaders have been discovered, showing that many of them lacked their signature accessories in the game's planning stages. Roark had brown hair instead of red and didn't wear glasses, Gardenia didn't have her black headband (which seems to confirm that the black part of her hair is a headband), Maylene's hair was shorter and she didn't have a nose bandage, Crasher Wake had a different mask that covered his face and resembled a Swampert's head, Byron's hair was smoother and less spiky, Candice had blue eyes instead of brown, and Volkner had brown eyes instead of blue.
The starters' English names were to be Sproutle, Scorchimp, and Ploopil instead of Turtwig, Chimchar, and Piplup. Turtwig's category was also going to be the "Greenleaf Pokémon" instead of the Tiny Leaf Pokémon. At least one trailer for the Diamond and Pearl era of the anime was made with Turtwig's beta name.
There's an unused Key Item called the "Azure Flute" that, when played at the Spear Pillar, reveals a staircase leading to the Hall of Origin, where the player would encounter Arceus. The Azure Flute would have been distributed as a Mystery Gift, but the idea was deemed too convoluted, and Arceus was straight up given out through events instead. Interestingly, Arceus would have been encountered at level 80, whereas Arceus are always given out at level 100 to make them impossible to EV train. The original version of the event was later made properly accessible in Brilliant Diamond and Shining Pearl as an Old Save Bonus for players who have a completed Pokémon Legends: Arceus file.
In a rematch, Fantina's Banette knows Snatch. Snatch was removed from the series in Pokémon Sword and Shield and thus doesn't have any proper code associated with it, so it cannot be used at all. This is fixed in the 1.1.3 update, where Snatch is replaced with Payback.
Likewise, Ace Trainer Meagan on Route 228 has a Delcatty that knows Feint Attack, which is also unusable. Version 1.1.3 replaces it with Covet.
Artist William on Route 208 is inexplicably renamed Kevin when rematched.
B-Team Sequel: Brilliant Diamond and Shining Pearl are the first mainline Pokémon games to not be developed by Game Freak. As Game Freak was busy producing both Pokémon Legends: Arceus and Pokémon Scarlet and Violet for release the following year, the remakes were developed externally by ILCA; a Japanese developer better known for their support work on games like Metal Gear Rising: Revengeance, Yakuza 0, and Dragon Quest XI, and had also done the Pokémon HOME cloud storage app.
Content Leak: Early copies got out into the wild about two weeks before the intended release date. This inadvertently revealed that the games shipped incomplete, as they were missing a fair amount of content (e.g. the opening cinematic, much of the postgame) that was part of the then-unreleased day 1 patch.
Dueling Games: With Shin Megami Tensei V, another game in the Mons genre that released a week earlier. They also represent radically different approaches to the genre, with Pokémon being an all-ages game with accessible gameplay while Shin Megami Tensei is Mature rated and features Nintendo Hard gameplay.
Fandom Nod: In the original Gen IV games, Bidoof and Bibarel are able to learn most of region's HMs, meaning they quickly gained a position on every player's team as the designated HM user and little else. Here, with in-party HM users now being replaced by summonable Pokémon, the remakes acknowledges this by having every Pokémon summoned to assist you with the Pokétch Hidden Moves app either be Bidoof and Bibarel, with the exception of the moves Fly and Defog.
God Never Said That: It's commonly stated that Pokémon Legends: Arceus started life as a proper Diamond and Pearl remake until Game Freak realized they had strayed away too far from the original games, so they decided to farm Brilliant Diamond and Shining Pearl out to ILCA while continuing to work on Legends: Arecus as an original title. Though PLA did enter development in late 2018, while BDSP only began development in early 2020, this reasoning has never been confirmed by official sources at Game Freak or The Pokémon Company.
Milestone Celebration: Brilliant Diamond and Shining Pearl were released during the 25th anniversary of the Pokémon franchise, which was also the year of the fifteenth anniversary of the original Diamond and Pearl.
Urban Legend of Zelda: Despite many people proving the rumor to be false, a good amount of fans still believe that the Pokédex will tell you if a roaming Pokémon is shiny before you encounter it. This can lead to players resetting a file with a shiny Mesprit or Cresselia simply because its sprite didn't show up as shiny in the Dex entry prior to encountering it.
What Could Have Been: The updated and unused description for the Park Ball suggests it was initially intended to return and be repurposed for Ramanas Park (which replaces the Pal Park from the original) before the idea was scrapped and the ball ended up Dummied Out.
◊ of the Sinnoh Gym Leaders have been discovered, showing that many of them lacked their signature accessories in the game's planning stages. Roark had brown hair instead of red and didn't wear glasses, Gardenia didn't have her black headband (which seems to confirm that the black part of her hair is a headband), Maylene's hair was shorter and she didn't have a nose bandage, Crasher Wake had a different mask that covered his face and resembled a Swampert's head, Byron's hair was smoother and less spiky, Candice had blue eyes instead of brown, and Volkner had brown eyes instead of blue.
