Rocket's Hitmonchan.

If an attack does damage to Rocket's Hitmonchan during your opponent's next turn (even if Rocket's Hitmonchan is Knocked Out), flip a coin. If heads, Rocket's Hitmonchan attacks your opponent's Active Pokémon for double that amount of damage. (If Rocket's Hitmonchan takes 20 damage, it does 40 damage to that Pokémon.)