The Green Mystery Egg is one of six secret eggs for Stop 'n' Swop found in Banjo-Kazooie. When (CHEAT AMIDST THE HAUNTED GLOOM A SECRET IN THE BATHROOM) is typed in at the Sandcastle in Treasure Trove Cove, the Green Egg could be found in Mad Monster Mansion in the bathroom on top of Loggo.
In Banjo-Tooie (Xbox Live Arcade), when you got this Egg in Banjo-Kazooie (Xbox Live Arcade), it brings it to a Stop 'n' Swop menu in the game. If you bring the Egg to Heggy the Hen, it, you will get the Homing Eggs cheat code, instead of the Blue Mystery Egg that came in the original game.