The reality of the Pokémon world is everything that's involved in battle has the chance of death. That's why when the female trainer Tina learned that the Pokémon in her strange blue egg would need to go live and grow in the ocean after it hatches, she was relieved. It meant that it wouldn't be dying in a battle.
She wasn't, however, informed of the future hatcher's natural homing device.
It hatched while Tina was sleeping one night. A healthy Manaphy hatched right in her arms. She awoke seeing the cutie asleep, curled into her arms like a little nest. Struck with a warm feeling, Tina almost kept the little guy. She knew better, though. If it was to stay healthy and safe as it grew it needed to be in the ocean. She went to route 13 of Unova, her home country, and let it free. After a wave goodbye, the Manaphy swam off to live in the sea, forever remembering Tina's scent and welcoming warmth.
Months after, Tina challenged a wild Scolipede in Pinwheel Forest. It's Rollout attack missed her Pokémon and hit her. She coughed up blood and rolled over in pain after the impact. The Scolipede fled, unaware of the serious iternal damage done to Tina. Using what was left of her enegy, she released all of her pokemon and told them to "Have fun." She died shortly after the last Pokémon ran off, next to the river of Pinwheel Forest.
Miles away, Manaphy just got old enough to live anywhere. It used its homing skills to find a similar warmth, as well as a similar scent. It was off to find where it hatched. Going up the river of Pinwheel Forest, it finally found the familiar scent but it was...off. It seemed to be masked and faded. Manaphy surfaced only to see Tina dead. It's been two days and mushrooms have already started growing on her lifeless body. The Manaphy was saddened, but it didn't recede back to the ocean. It did exactly what any Manaphy would; it went back to its hatching ground by squeezing itself in between Tina's cold arms, using it's memory to fake warmth. Tears would fall from its eyes randomly as it stayed there. It only ever left to get food and water, only allowing the decay of time take away its hatching ground and, more importantly, it's mother.
"Born on a cold seafloor, it will swim great distances to return to its birthplace"-Diamond Pokedex Entry.