Created all the way back in 1987, Picross, more commonly referred to in other parts of the world as a Nonogram, is a grid puzzle with numbers. You're meant to fill out certain spaces in the grid to form a complete picture.
The Picross games started with a release on the original Game Boy titled Mario's Picross in 1995. Two follow-ups were later released in Japan only. It took a good few years later before there was ever another Picross game to be released outside of Japan. More recently, these games have been coming out for the Nintendo Switch under the Picross S series.
Before the release of Pokémon Picross for the Nintendo DS, there was a game that was announced for the Game Boy color as a successor to the second Picross game for the original Game Boy. This is so far the only Pokémon game to ever be canceled after it was announced. The reason for the cancelation of the game has never been made public, however, there is enough information for us to be able to take a look and see what this game was going to be.
There were multiple magazine articles and pictures released surrounding the game in Japan during the spring of 1999. With only the first generation of games being out at the time, only the first 151 Pokémon were expected to be included in the game.
In one of the updates that were released about the game's details, it was revealed that it was going to be compatible with the Game Boy Printer. This was another accessory to the Game Boy, much like the Game Boy Camera. The printer worked with thermal paper that included adhesive backing.
The Game Boy Printer was released in multiple colors, and as pictured above, there was a special Pokémon-themed printer that was a Japan-only exclusive. From the looks of it, the original Pokémon Picross was meant to be compatible with the accessory. This was a way to effectively promote the device and one could print the completed puzzles. However, the Game Boy printer was unfortunately not a successful accessory.
Shortly after these announcements, the idea of the game seemed to be abandoned altogether.
On April 1st of that same year, Nintendo released Picross NP Vol. 1 on the Super Nintendo through a Nintendo Power service. This was a Japan-only service that included a special Character Mode chapter in each installment that was released. The first one included 12 different Pokémon puzzles to complete.
It's worth noting that none of the puzzles featured in NP Vol. 1 are ones that were revealed in any magazine article. The other NP Volumes released featured other Nintendo franchises such as Legend of Zelda, StarFox, Donkey Kong, and etc.
Some years later, fans were finally able to see a released version of Pokémon Picross from the same developer, Jupiter Corporation, in December 2015. This game was free to start, featuring brand new puzzles, Pokémon abilities, and a Daily Training feature. However, there were requirements that made this game difficult to unlock new areas unless you invested money in it.
As it stands, Nintendo very rarely announces a game that ends up being canceled. They're very confident in their announcements, and are very diligent in updating fans even if it includes delays. There could be countless other Pokémon games that were left on the cutting room floor, but this game was very much announced in Japan only to disappear without a trace. It's a rarity from the Big N, and definitely something we're not likely to see again for a while.