From Bulbapedia, the community-driven Pokémon encyclopedia.
Magazine scan showing information on the game
Pokémon Picross (Japanese: ポケモンピクロス) is an unreleased puzzle game for the Game Boy and Game Boy Color announced in various game magazines in Spring 1999. A picross, from picture and crossword, is a genre of puzzle games, also known as a nonogram.
Though ultimately unreleased, the SNES game Picross NP Vol. 1 released later that year included 12 Pokémon puzzles.
The international versions of Pokémon HeartGold and SoulSilver featured a game similar to picross at the Game Corner, Voltorb Flip.