It is a legitimate game in the Pokémon Mystery Dungeon series released for the Nintendo DS in 2009. It's a popular and well-regarded game, known for its compelling story, challenging gameplay, and unique setting.
If you have any concerns about the game's legitimacy, you can easily verify it by looking for information about it on official Nintendo websites, game review websites, or by checking if your copy of the game has the proper packaging and markings.