Mystery Egg is an item in the “Plot advancement” category (Key Items).
In-game descriptions
| HeartGold / SoulSilver · Black / White · Black 2 / White 2 | A mysterious Egg obtained from Mr. Pokémon. What’s in the Egg is unknown. |
|---|---|
| X / Y · Sun / Moon · Ultra Sun / Ultra Moon · Let’s Go, Pikachu! / Let’s Go, Eevee! · Sword / Shield | A mysterious Egg obtained from Mr. Pokémon. What is in the Egg is unknown. |
Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h