The Mystery Egg (Japanese: ふしぎなタマゴ Mystery Egg) is a Key Item introduced in Generation II. As its name implies, it is an unusual egg found by Mr. Pokémon.
The Mystery Egg is given to Professor Elm for him to study. After defeating Falkner and obtaining the Feather Badge, one of his aides will return the egg to the player, revealing that is in fact a Pokémon Egg. The Egg, if the player chooses to keep it in their party, will eventually hatch into a Togepi.