In Spring of 1999, a new addition to the nonogram puzzle game series Picross was announced and widely advertised/featured in Japanese magazines. Titled Pokémon Picross, the game was being developed for the Game Boy Color, and would most likely have been simply a Pokémon themed version of Picross.
Allegedly intended as a "spin-off" of the hugely successful Pokémon Game Boy titles, Pokémon Picross, for whatever reason, never saw the light of day.
Notably, on April 1st, 1999, a Japan-only downloadable SNES title, Picross NP Vol. 1, was released, featuring twelve Pokémon puzzles. It is largely assumed that Pokémon Picross was either reworked into Picross NP Vol. 1, or that whatever had been finished of the game was implemented into it.
However, on December 3, 2015, Pokémon Picross was released on the Nintendo 3DS eShop. It doesn't really play like the cancelled GBC version though, but there are similarities.