From Bulbapedia, the community-driven Pokémon encyclopedia.
Easter eggs are hidden parts of a game or software that are usually discovered by more experienced players. There have been a number of Easter eggs throughout the Pokémon franchise. In the games, there are several items, locations and events that are considered by some to be Easter eggs.
In the core series
A major Easter egg in the game is the truck from the Generation I games. In Pokémon Red, Blue, and Yellow, as well as their remakes Pokémon FireRed and LeafGreen, there is a truck which appears in the S.S. Anne's port which serves no evident purpose except as scenery. With scenery being very sparse in the Generation I games, and this being one of the few sprites with no other purpose than to exist in one spot, fans began speculating that another purpose existed for this truck, and that it could perhaps be moved with Strength. Thus, a popular rumor sprang up that a Poké Ball containing Mew could be found underneath. The only flaw with this was that the HM for Strength could only be obtained after getting HM01 (Cut), which required the S.S. Anne leaving. There were a number of ways that players went around this problem. The first, and most obvious was trading a Pokémon that knew Cut into their game. The second involved getting the HM from the ship's Captain then battling a Trainer and losing, having them run to the Pokémon Center without the ship leaving. They could progress through the game until Surf and Strength were acquired and then attempt the truck. The rumor was proven false.
Later, in the Generation III remakes of Red and Blue, the truck is still present, however, there is actually something of a point to get to it this time around. On its pier, if the player walks down a bit, they can find a hidden Lava Cookie, unavailable until later in the game at the Sevii Islands. The truck had been positioned there, as an Easter egg, due to the old, popular rumors that were spreading during the time of Pokémon Red, Blue, and Yellow.
Diamond dust in Snowpoint
Diamond dust (Japanese: ダイヤモンドダスト diamond dust) is a special variation of snow that has appeared in select locations and at select times in core series games since Generation IV. Diamond dust has no effect in battle.
On January 10, 2008, Director Junichi Masuda announced on his blog that fans should play Pokémon Diamond and Pearl on January 12, his birthday, for a surprise. The surprise was discovered to be sparkling diamond dust falling in Snowpoint City instead of the normal, light snow.
Below are the known dates and locations that diamond dust occurs in the Pokémon games.
Another Easter egg in the games is secret wallpapers for the Pokémon Storage System that can only be unlocked via outside resources in Pokémon Emerald, Pokémon Diamond, Pearl, and Platinum, and Pokémon HeartGold and SoulSilver. A special phrase must be entered in-game to unlock these wallpapers, with the phrase depending on the player's ID number. Online generators at places like Filb.de can be used to obtain these phrases.
In Pokémon Emerald, the player may speak to the father of a sick girl named Walda in Rustboro City to obtain a "Friends" wallpaper by telling him a word that will make Walda laugh. It is possible to unlock a variety of wallpapers, with different combinations of background and patterns and colors, by telling the father different words, but only one Friends wallpaper may be unlocked at a time.
In the Generation IV games, eight wallpapers can be unlocked. Unlike in Emerald, all eight wallpapers can be unlocked at the same time, each requiring its own unique phrase. These wallpapers differ between the Diamond and Pearl, Platinum, and HeartGold and SoulSilver games. In Diamond, Pearl, and Platinum, the player may speak to a TV producer at Jubilife TV to unlock the wallpapers. In HeartGold and SoulSilver, the player may speak to Primo in the Violet City Pokémon Center to unlock them.
In the spin-off games
In Pokémon Stadium, when the player uploads a Pikachu from Yellow, it will be voiced by Ikue Ohtani and have unique sound effects. Likewise, if a Pikachu or Raichu that knows Surf is used, it will have a unique attack animation. This animation also occurs in Pokémon XD and Pokémon Battle Revolution.
Super Smash Bros.
An Easter egg can be found in Super Smash Bros. Brawl while the player is playing in Pokémon Stadium 2 stage when the stage is in Ice Mode. If the player angles the camera and looks through the window of the cabin that Snorunt is in, a poster of a pet cat can be seen on the wall. The cat is rumored to belong to Sakurai, but he announced that it is false.
In Pokémon Play It!, only two decks are normally available: the Fighting Deck and the Water Deck. If the player types "FIRE", two additional decks become available as well: the Fire Deck and the Xtra Deck. The additional decks are simply unlocked with no feedback or confirmation. From this point onward, they appear at the list of available decks whenever the player starts a new game. However, when the player closes the game application or accesses the "Extra" option (which also closes the game application and opens a separate application), the additional decks become unavailable as before. Whenever the player opens the game, they will have to type "FIRE" if they wish to use the additional decks.
Specifically, this shortcut works at any point during the main game, at the main menu or any the "Learn" options; it does not work during the initial cinematics or if the player chooses the "Extra" option (which closes the game as stated above). If the player types "FIRE" when the list of available decks is displayed with only the two initial decks, the code works as usual but the list of decks will not be updated until the player starts a new game (without closing the game application).
Within the coding of the Mobile Trainer cartridge (the main Game Pak packaged with the Mobile Game Boy Adapter) is a reference to an alleged 'game' called "Pokémon Moss" (Japanese: ポケットモンスター苔). The same reading with different kanji (虚仮) can also be interpreted as "Pokémon Fool", hence it is possible this was simply a joke by the developers to trick the fanbase.