The Mystery Eggs are a series of mysterious, collectible eggs that are most often used for unlocking extra features.
Originally, six Mystery Eggs and the Ice Key would have been involved in the scrapped Stop 'n' Swop feature, where the player would transfer the items from Banjo-Kazooie into Banjo-Tooie. The Mystery Eggs have no role in Banjo-Kazooie aside from appearing in a Stop 'n' Swop menu.
Eventually, in Banjo-Tooie, half of the Mystery Eggs (pink, blue and yellow) and the Ice Key appeared as collectible items for unlocking content within the game itself. In the Xbox Live Arcade port, the in-game Mystery Eggs were respectively replaced with the Bronze, Silver, and Gold Mystery Eggs.
The Stop 'n' Swop feature was later added in the Xbox Live Arcade ports for both games. As a result, the Mystery Eggs were used to unlock content between the two games, including Mumbo Crates with a vehicle part in Banjo-Kazooie: Nuts & Bolts.
Pink Mystery Egg
In Banjo-Kazooie, the Pink Mystery Egg appears in the sunken Sharkfood Island of Treasure Trove Cove, which can be raised out of the ground by entering the relevant Sand Castle cheat, "CHEAT OUT OF THE SEA IT RISES TO REVEAL MORE SECRET PRIZES." Banjo and Kazooie must jump between many alcoves on a spiraling hill and obtain the Pink Mystery Egg at the top. In the ending, after obtaining 100 Jiggies, Mumbo Jumbo shows a photograph of the Pink Mystery Egg, hinting at how the duo can obtain it.
In the Banjo-Tooie, the Pink Mystery Egg appears behind a grate high up in Spiral Mountain. To break it, Banjo and Kazooie by must fly high up and throw a Grenade Egg to break open the grate. They can then enter the alcove and defeat the Banjo-Kazooie Game Pak enemy to obtain a Pink Mystery Egg. The duo must return it for Heggy to hatch, and as a result unlock the Breegull Bash ability. The Pink Mystery Egg was replaced with the Bronze Mystery Egg in the Xbox Live Arcade remake.
The Pink Mystery Egg cannot be directly obtained in the Xbox Live Arcade port of Banjo-Tooie, and was replaced by the Bronze Mystery Egg. It can only be obtained from Banjo-Kazooie on Xbox Live Arcade. If the player has a Nuts & Bolts save file, they can unlock the Fluffy Dice vehicle part.
Blue Mystery Egg
In Banjo-Kazooie, the Blue Mystery Egg appears behind a locked door in Gobi's Desert, where Gobi is located before leaving the location. This door can only be opened by spelling out "CHEAT A DESERT DOOR OPENS WIDE ANCIENT SECRETS WAIT INSIDE" from the Sand Castle. By accessing the room, the duo must use Beak Buster on a ground switch to open a sarcophagus in the proceeding room with the Blue Mystery Egg. The egg's location is also hinted at in one of Mumbo's photographs in the 100% completion ending.
In Banjo-Tooie, the Blue Mystery Egg is guarded by a Banjo-Kazooie Game Pak enemy in the "Behind the Waterfall" region of Spiral Mountain. After obtaining the Mystery Egg, Banjo and Kazooie must return it for Heggy to hatch. This allows the duo to unlock the "Homing Eggs" cheat from the code chamber of Mayahem Temple. This egg was replaced with the Silver Mystery Egg in the Xbox Live Arcade port.
In the Xbox Live Arcade remake of Banjo-Kazooie, the obtained Blue Mystery Egg can automatically be transferred to Banjo-Tooie on Xbox Live Arcade or Nuts & Bolts. In the latter game, the Blue Mystery Egg unlocks a Mumbo Crate in Showdown Town with the Goldfish vehicle part.
Yellow Mystery Egg
In Banjo-Kazooie, the Yellow Mystery Egg appears during the wintertime in Click Clock Wood, on a table in Nabnut's house. For the Yellow Mystery Egg to appear there, Banjo and Kazooie must enter the Sand Castle and then spell out "CHEAT NOW BANJO WILL BE ABLE TO SEE IT ON NABNUTS TABLE."
In the original Banjo-Tooie, the Yellow Mystery Egg is on a nest on the upper floor of Heggy's Egg Shed. Kazooie must hatch the egg herself because Heggy cannot reach it. After doing so, the player unlocks Jinjo as a playable character in multiplayer mode.
Cyan Mystery Egg
In Banjo-Kazooie, the Cyan Mystery Egg appears in the cellar of Mad Monster Mansion, in a barrel marked "1881" and sealed with an X. This barrel can be opened by entering "CHEAT DONT YOU GO AND TELL HER ABOUT THE SECRET IN HER CELLAR" from the Sand Castle. The Cyan Mystery Egg sits on a small circle platform surrounded by a small splash of water. Banjo and Kazooie are not required to enter a cheat in the Xbox Live Arcade port.
In the Xbox Live Arcade port of Banjo-Tooie, if the Cyan Mystery Egg was obtained in the Banjo-Kazooie port, Banjo and Kazooie can it to Heggy's Egg Shed. Heggy hatches the egg, and the player unlocks the Stop 'n' Swop II submenu. In Banjo-Kazooie: Nuts & Bolts, obtaining the Mystery Egg spawns a Mumbo Crate containing the Mole-on-a-Pole vehicle part on a rooftop in the theatre district.
Green Mystery Egg
In Banjo-Kazooie, the Green Mystery Egg is located above Loggo in a bathroom of Mad Monster Mansion, after Banjo and Kazooie spell out "CHEAT AMIDST THE HAUNTED GLOOM A SECRET IN THE BATHROOM" in the Sand Castle.
In the Xbox Live Arcade remake, obtaining the Green Mystery Egg unlocks a Mumbo Crate in Banjo-Kazooie: Nuts & Bolts; it contains the beacon vehicle part.
Red Mystery Egg
In Banjo-Kazooie, the Red Mystery Egg appears on the bed of the captain's quarters in Rusty Bucket Bay. This room can be entered by breaking one of the windows.
In the Xbox Live Arcade remake, obtaining the Red Mystery Egg spawns a Mumbo Crate with a disco ball vehicle part in Banjo-Kazooie: Nuts & Bolts. It is located behind a fence, which can only be destroyed with the Laser, in the Uptown area of Showdown Town in Banjo-Kazooie: Nuts & Bolts. A fake Red Mystery Egg also appears in Banjoland, which contains a Gruntbot if Banjo and Kazooie break it open.
In addition, when the Red Mystery Egg is transferred to Banjo-Tooie on Xbox Live Arcade, Heggy can hatch it for the player to unlock a Banjo-Kazooie theme on the Xbox 360.
Gold Mystery Egg
The Gold Mystery Egg replaces the location of the Ice Key in the original Banjo-Tooie. Unlike the Mystery Eggs originating from Banjo-Kazooie, the Gold one does not have question marks on it, but is covered in white spots instead. The Mystery Egg is only obtainable after unlocking the Stop 'n' Swop II submenu, which can be done if Heggy hatches the Mystery Eggs originating from Stop 'n' Swop.
Silver Mystery Egg
The Silver Mystery Egg replaces the role of the Blue Mystery Egg from original Banjo-Tooie. Unlike the Mystery Eggs from Banjo-Kazooie, the Silver one is yellow dotted and have no question marks printed on them. Like the Gold and Bronze Mystery Eggs, the Silver one is only obtainable after unlocking the Stop 'n' Swop II menu.
Bronze Mystery Egg
The Bronze Mystery Egg replaces the role of the Pink Mystery Egg from the original Banjo-Tooie. It is also white-spotted like the Gold Mystery Egg. It can only be obtained after the player unlocks the Stop 'n' Swop II menu.