From the Super Mario Wiki
Glowing Rabbits are rare, shiny types of rabbits that appear in Super Mario 64 DS. When eight Power Stars are collected, Glowing Rabbits will start to randomly appear around the castle in place of the character's other rabbits. Eight of the rabbits must be caught to earn a Star. When all eight rabbits are caught, a key will be obtained and it can unlock the last door in the Character Selection which contains a secret Power Star. Glowing Rabbits will also not longer appear.