I smell a horse.
A play on the phrase "I smell a rat" --- a "rat" in the figurative mobster context refers to a police informant that has infiltrated the group. Instead of a figurative rat, here, the mob boss smells a literal horse, which is behind him. Humorously, the horse (whose disguise is only a pair of sunglasses) is indeed a figurative rat, because it's affiliated with the police --- there's a cop overtly riding on it, and it's wearing a police outfit.