The user drains the target's energy with its horns. The user's HP is restored by half the damage taken by the target.
Horn Leech does damage to the target, then restores the user's HP by up to half the damage dealt to the target. In a Double Battle, Horn Leech can target any Pokémon around the user. In a Triple Battle, Horn Leech can only target a Pokémon that's adjacent to the user.