From the Super Mario Wiki
Glowing Rabbits are rare, shiny types of rabbits that appear only in Super Mario 64 DS. These rabbits appear to be white with sparkles appearing from their body. When 50 Power Stars are secured, Glowing Rabbits will start to randomly appear around the castle in place of the character's other rabbits, but they are so scarce, it's rare to find one. If the player uses Yoshi, he can catch six of the rabbits outside in front of the castle. He can also get the other two if the player goes back in and outside the castle repeatedly until the final two glowing rabbits appear. There are eight Glowing Rabbits in total. Once all eight of them are caught, the player is rewarded with a key that unlocks the last door in the character selection room, which obtains a secret Power Star. Boo laughs will be heard if the player enters the door again after entering it for the first time. Glowing Rabbits no longer appear after finding all of them.
The final Glowing Rabbit caught states that it is the eldest of the Glowing Rabbits. Additionally, they appear to be related to each other and are all male, as each rabbit then before the last one states that it is the "second fastest of [their] eight brothers."
If a rabbit before the eighth rabbit is caught, the player gets a chance to save their progress.