The Game Boy was a cheap and easy system to develop games for, often with very short development cycles. This, combined with its massive userbase, meant very few games were announced and later cancelled.
However, a handful slipped through late in the Game Boy's life as development priorities shifted from the Game Boy Color to the Game Boy Advance.
A 2D platformer set largely in a sprawling underwater land.
The game was developed by Tose, a Japanese studio whose remit is to assist developers in creating games without credit or keeping any of the rights. Legendary Starfy was significant in that it was the first game they held rights to, jointly with Nintendo.
The game was announced late in the Game Boy Color's life, and development was shifted to Game Boy Advance.
See the Legendary Starfy GBA page for more details on the game.
An RPG that made use of moves called Gimmicks (similar to the action moves in the Paper Mario series) to carry out attacks. As Demille, you would have to fight through a world of weird, colourful characters to rescue his girlfriend from the evil Prince Tomato.
The game was developed by Alphadream, a company set up by former Squaresoft employees. Nintendo approached the company after they released their first RPG for the Game Boy, Koto Battle.
As with Legendary Starfy, the game was announced late in the Game Boy Color's life and ended up being redesigned and released on Game Boy Advance as Tomato Adventure.
A pocket version of Jet Force Gemini was being worked on by the Rare Game Boy development studio, who were extremely active on the Game Boy Color.
However, perhaps due to the poor performance of Rare's self-published titles Perfect Dark and Conker's Pocket Tales, the game was never announced and development was stopped. A development cartridge featuring some areas from the game was later uncovered.
The Legend of Zelda: Oracle of Ages and Oracle of Seasons were originally planned to be part of a connected trilogy of games. One was going to be a Game Boy Color remake of the original Legend of Zelda, while the other two were planned to be new games. By connecting them with each other you could trade items and unlock new areas of the game.
However, early on it was decided that three games would pose too many problems at the testing stage, and the three games were merged into two. Many of the "remade" features from the missing Oracle game survived in the existing games.
A Game Boy Color remake of the original Game Boy adventure For The Frog The Bell Tolls was announced to be in development. Sadly, the project was never shown and was quietly cancelled.
Please note that while some screenshots of the Color game can be found on the internet, these were fan-made mockups that appeared long after the game was announced.
A rhythm game that was to feature a character named Barbara the Bat. While the game was in development on the Game Boy, it was decided that the system didn't feature enough buttons to make the game a viable competitor in the rhythm action genre.
Development was shifted to the Game Boy Advance, but despite screenshots being shown, the game was held back again and eventually arrived as Great Concert! Band Brothers, a launch title for the Nintendo DS.