I got this idea from watching NCIS episode Beast master.
Percy was fighting a snake woman drawing a double edge sword. The monster dodged and wrapped her tail around the son of Poseidon and began to squeeze the light out when he heard the hooves of a police horse coming toward him. "Freeze! Put the boy down!" A park policeman yelled to get the woman's attention as he drew out his gun.
The horse began to bunk around and his partner flew from her and the human partner headbutted the snake woman, who uncurled her tail, allowing for the son of the sea god to land on the floor as the horse tackled and reared its front to hooves to meet the face of the woman moaning in agony. Percy than stabbed his blade into the woman, disappearing into golden dust as the policeman began to wake up.
"What… happened?" The police asked as Percy thought of something random. "Your horse saw a cat race by and got spoked." Percy than turned and telepathically thanked the horse as she bowed to him, stunning her human partner. "You are welcome Prince of the Sea. It was a great honor to save you from that serpentine."