It's a special event egg that can only be obtained through certain events, and it's not available in the regular game. If you're playing a normal copy of Diamond, you won't be able to get a Manaphy egg.
Here's why it's special:
* Manaphy is a Mythical Pokémon: This means it can't be found in the wild in the game.
* It hatches into a Manaphy: This Pokémon is known as the "Sea Pokémon" and is a powerful Water-type.
If you're interested in getting a Manaphy egg, you'll need to look into events that happened in the past or use specific methods (like connecting to another game) that are no longer possible.