Ninja Gaiden Shadow, "Shadow Warriors" in Europe and Australia, for the Gameboy was not originally planned to be a Ninja Gaiden game. The game was actually going to be a port of the NES game "Shadow of the Ninja" developed and published by Natsume, but Tecmo bought the rights to the game and was reworked to be a Ninja Gaiden prequel.
Contributed by Combat Lobster