The Cyan Mystery Egg is a secret egg obtained for Stop 'n' Swop in Banjo-Kazooie after entering the special cheat code. It is one of the two eggs located in Mad Monster Mansion. It can be found in the cellar. Hint: 'X' marks the spot!
In Banjo-Kazooie (Xbox Live Arcade), no cheat code is necessary to access the barrel in the cellar of Mad Monster Mansion, although it still has no effect in the game.
In Banjo-Tooie (Xbox Live Arcade), if you have collected the egg in Banjo-Kazooie, it can be taken to Heggy's Egg Shed where the egg can be hatched. This unlocks Stop 'n' Swop II, which includes 7 new awards for the player to obtain, not found in the original Banjo-Tooie.
In Banjo-Kazooie: Nuts and Bolts, if the player has collected the egg, a crate will appear on a rooftop in the theatre district. It contains Mole-on-a-Pole.
The Cheat code to make the barrel open is ( CHEAT DONT YOU GO AND TELL HER ABOUT THE SECRET IN HER CELLAR )