When you get the shadow ferret from Cali how come it wouldn't let you battle at the colosseum in Pokemon Colosseum for nintendogamecube?

In Pokémon Colosseum, the Shadow Ferret, also known as Shadow Zangoose, cannot be used in battles at the Colosseum because it is a Shadow Pokémon. Shadow Pokémon are initially unable to compete in certain battle formats until they are purified. To use the Shadow Ferret in battles, players must first complete the purification process, which involves battling with it and using special items or visiting a purification chamber.