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DP box, often called the Generation IV Missingno. due to its appearance, is a glitch Pokémon in Pokémon Diamond and Pearl.
It will appear in a player's party if they attempt to enter a double battle with two fainted Pokémon, an egg, and a healthy Pokémon in that order in their party. When sent out, it will have Bulbasaur's cry and will sparkle as if it were a shiny Pokémon. It has no attacks, and will not appear if the party is looked at in battle. It is also obtainable in the wild by using the japanese action replay.
It has no official name (as there is no data, its name appears as simply blank space), so it is simply called by fans "DP box" due to its appearance as a simple white rectangle with two black lines running horizontally across it. This blocky appearance also causes many fans to hearken back to the appearance of Missingno. in Pokémon Red and Blue.
It only appears in the Japanese version, due to many of the glitches being removed as the games were translated for the American audience.
External links
YouTube video of its appearance in battle