The Manaphy Egg was received through an event that took place in the Pokemon Ranger games, and it took place in various places around the times of 2006-2007. So you're about 7 years late, nothing much.
>In Pokémon Ranger, the Manaphy egg washes ashore on a beach, where Murph picks it up. When he takes it to Professor Hastings, several Go-Rock members steal the Egg, resulting in a search for it. After the player safely returns the egg, Hastings reveals that the egg can only hatch if it is given to a Trainer in a faraway region.
You could transfer it to one of your Gen IV games, and hatch it in those games. At the time the only Gen IV released were Diamond and Pearl. It had a different egg sprite, and it had a special animation when you hatched it. It is now obsolete and you cannot get it legally. It also should be noted Manaphy could also breed a Pokemon called Phione, but the Phione could not evolve into Manaphy.
Hope I helped. :)
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