A Bad Egg (stylized as Bad EGG in Generation III) is a phenomenon present in the Generation III Ponymon games that results from a corruption of Ponymon data so that the checksum does not match up with the data's calculations.
Bad Eggs are not really Ponymon Eggs, and may possibly never have been, but is the default message returned by the game if the checksum is off (which happens only if data is altered badly). Bad Eggs rarely hatch and cannot be released, merely taking up space, though there are methods of removing them. It is possible to trade a Bad Egg away into another game, or remove it by cheating.
Like a normal Egg, Bad Eggs appear with a type of ???, as well as have the same Ponymon status screen as an Egg would. Despite this, the similarities end there, as Bad Eggs can hold items (such as Pony Balls, which cannot be taken), be placed within types of Pony Balls aside from the standard kind, and have Seals attached (which prevents them from being deposited in the PC). After that, it is only possible to remove the seal from the Bad Egg by depositing it into the Ponymon Day Care. Some may be reported to have Ponyrus or be cured of it, and may be fainted.
A Bad Egg placed in the first slot of the party will, rather than having a normal Egg menu sprite, have a differently colored version of the menu sprite of the Ponymon following it, such as a golden YApplejack, a blue The_Doctor, or a brown CHRYSALIS. It is unknown exactly why the sprite retains the last Ponymon's sprite, but the color change is due to the use of palette 0 for this sprite. This is why Ponymon with palette 0 do not change color.
If it is forced to hatch through use of a cheat code, a ? will come out and the game will immediately freeze.
Appearances of Bad Eggs
When using the Cheat Codes code for quick Day Care level-up, an invisible Bad Egg will appear in the party, which can be switched around using the PC and used in battle. Sometimes, the Bad Egg will take form of the silhouette of a Ponymon in the PC Box. However, attempting to view its summary screen will cause a game freeze.
Alternatively, by using codes to capture wild Ponymon instantly, reducing their HP to 0 yet keeping the battle going, it will be sent to the PC instead as a Bad Egg. This Bad Egg can be removed from the game by picking up another Ponymon while it is held, then setting it back down. This Bad Egg appears differently; instead of an Egg in its status box, it is the "unseen Ponymon" image used in the Ponydex and by ?????????? (also known as Decamarks). Like Decamarks, using it in battle will cause an instant white out if used without any other Ponymon in the party.
Bad Eggs can also occur if the player attempts to hack a Ponymon which has an illegal moveset, as the checksums will not add up correctly.