Pokémon Yellow

Pokémon Yellow is basically Red and Blue, reworked to match the anime more closely. This includes that one yellow mouse everyone loves... right?

"Hurry, get away!"
An unused battling system of identifier 03 exists in the single byte managing what battle system is used (normal, Safari Zone, Old Man, etc.), but is never used in the final releases. Attempting to attack, switch Pokémon, or use an item will give the message "Hurry, get away!" The chance of escaping from a battle in this mode is always 100%.

As this battle mode only exists in Yellow, it could be suggested that Game Freak originally intended players to receive a Pikachu past Pallet Town themselves. If so, this battle system would be essential to prevent the player from sending out an invalid Pokémon of identifier 00 (3trainerpoké) with an invalid moveset corresponding to information from other existing variables in the game. Instead, a battle system was implemented so that Professor Oak would capture a Pokémon (which always has the cry "Pika!")

The battle mode can be triggered by using GameShark code and triggering a battle encounter.

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Unused Music
An unused track present only in Yellow, located in the soundbank of the Pikachu's Beach minigame and the game's extra features. It may be related to the "Hurry, get away!" battle system, although probably not the battle system itself because battle music is located in a separate soundbank.

The track can be played by enabling GameShark codes and accessing a different map.

Pikachu's Beach 750-Point Bonus
The Pikachu's Beach minigame is available via the Dude inside of a house at Route 19, south of Fuchsia City, which only exists in Yellow. Talking to the Dude with a Pikachu knowing the move Surf with the same O.T. and Trainer ID as the player in the party (available via Pokémon Stadium) unlocks a minigame similar to Excitebike where Pikachu has a limited amount of time (HP) to reach a shore and get marked based on remaining HP and the total number of "radness" points earned by flips.

There is a radness bonus of 750 points (internal index number of ) but it isn't assigned to any number of flips, regular or "special" (forward-backwards), in the game. It can be accessed with GameShark code. GameShark code can be used to replace the first Pokémon in the party's first move with Surf.

Red, Green, and Blue Leftovers
All the unused data from Red, Green, and Blue remains in the programming code of Yellow. There is a small change pertaining to the unused Butterfree-for-Beedrill in-game trade, however: its English nickname was changed from "CHIKUCHIKU" to "STINGER". It otherwise remains unavailable in regular gameplay.

Regional differences
The Japanese Yellow was a Super Game Boy game, just like Red and Blue. Game Boy Color support was added to the international versions.

Unused Cries
There are multiple unused cries for Pikachu that are various voice types performed by Ihue Ohtani, the Japanese voice actress of Pikachu in the anime.

Unused Gift Pokémon Function
The unused debug function to add Pokémon to the player's party from Red and Blue also exists in Yellow, at for all versions of Japanese Yellow,  for English Yellow,  for German,  for French,  for Italian and  for Spanish. In all versions of Yellow, four Pokémon are added to the player's party:
 * Snorlax, level 80
 * Persian, level 80
 * Jigglypuff, level 15
 * Pikachu, level 5

Unused Trades
In addition to the aforementioned Butterfree-for-Beedrill in-game trade, Yellow adds two more unused trades, probably related to the removal of the in-game trades in Cerulean City and Vermilion City: Pidgeot-for-Pidgeot and Mew-for-Mew, respectively. In the Japanese Yellow, both received Pokémon are nicknamed まつみや (Matsumiya, presumably after Toshinobu Matsumiya, who is listed in the credits of Yellow under Game Scenario).

The English localization keeps the unused trade data, but changes the names of the received Pokémon to "MARTY" and "BART", respectively.