Gyarados

Upon entering battle or receiving the ability, the opponent’s Attack lowers one stage. In a Double Battle, both opponents’ Attack are lowered. Pokémon with the Clear Body, Hyper Cutter, or White Smoke ability are unaffected. In a link battle, if both sides switch on the same turn, and first player sends out a Pokémon with Intimidate, the opponent’s Attack will be lowered before the opponent’s Pokémon switches.

Attack is raised by one stage when the Pokémon knocks out another Pokémon