The Ability reduces damage done to both the player's Active and Benched Pokémon, as it does not specify which Pokémon on the player's side gets affected.[1][2]
If the opponent's Active Pokémon uses an attack whose damage isn't affected by any effects on the Defending Pokémon (eg. Shred, or using the Supporter cardPhoebe on a Pokémon VMAX), the damage done is still reduced by Intimidating Fang, as the Ability's effect applies for the Attacking Pokémon and not the Defending Pokémon.[1][2]