In the Canalave Library there is a Book that was not in the original games for the DS, called the "The Sea's Legend", that the game describes as being "recently discovered" and that states the mysterious following text:

Spoiler:"Once upon a time in the East Sea, there was a Pokémon known as the prince. A brave human asked Pokémon living in the sea to let them see the prince. Mantyke, Buizel, and a Quilfish with huge spikes acknowledged the human's bravery and joined them. Together, they set off in a boat over the sunset-streaked sea, sailing through the ocean gate stretched over the waves. News of this reached the ears of the prince, who went to meet the brave little party at the Seaside Hollow."

It's possible that this text was put into Brilliant Diamond and Shining Pearl Spoiler:to tease the then upcoming game Pokémon Legends: Arceus via giving the player a hint of a storyline or event that may happen in the latter game's plot. Furthering the suspicious nature of the text, Game Freak, when sending out review copies of the games, forbid any reviewer from talking about any of the text in the library.