Once during your turn (before your attack), you may attach an Evolution card from your hand to Brock's Ninetales. (This doesn't count as evolving Brock's Ninetales.) Treat Brock's Ninetales as if it were that Pokémon instead. It can't evolve, devolve, or use the Pokémon Power of that Pokémon. During your turn, you may discard the Evolution card attached to Brock's Ninetales. This power can't be used if Brock's Ninetales is Asleep, Confused, or Paralyzed. If Brock's Ninetales becomes Asleep, Confused, or Paralyzed, discard all Evolution cards attached to it.
The Japanese print initially allowed Brock's Ninetales to use the Pokémon Powers of the card that was attached to it via Shapeshift. This was later errata'd in Japan, and the errata was included in the international releases of the card. The Shapeshift power was still a rulings nightmare, similar to Ditto, despite the errata.