Erratic Mutation

Oracle Text

Choose target creature. Reveal cards from the top of your library until you reveal a nonland card. That creature gets +X/-X until end of turn, where X is that card's converted mana cost. Put all cards revealed this way on the bottom of your library in any order.

Card Rulings

2/1/2007 The target creature is chosen as the spell is cast. You don’t reveal cards until the spell resolves.