If not for the horses, Piper would've died.

Gods of Olympus.” Piper stared at Leo. “What happened to you?”
His hair was greased back. He had welding goggles on his forehead, a lipstick mark on his cheek, tattoos all over his arms, and a T-shirt that read HOT STUFF, BAD BOY, and TEAM LEO.
“Long story,” he said.

Piper and Hazel were ready to go, but first Annabeth turned to Percy, who was leaning on the starboard rail, gazing over the bay.
Annabeth took his hand. “What are you going to do while we’re gone?”
“Jump in the harbor,” he said casually, like another kid might say, I’m going to get a snack.

Festus just detected a large group of eagles behind us—long-range radar, still not in sight.”
Piper leaned over the console. “Are you sure they’re Roman?”
Leo rolled his eyes. “No, Pipes. It could be a random group of giant eagles flying in perfect formation. Of course they’re Roman!

Blackjack," Percy said, "this is Piper and Jason. They're friends."
The horse nickered.
"Uh, maybe later," Percy answered.
Piper had heard that Percy could speak to horses, being the son of the horse lord Poseidon, but she'd never seen it in action.
"What does Blackjack want?" she asked.
"Donuts," Percy said. "Always donuts.

Percy blinked. “So your brother is a winged horse. But you’re also my half brother, which means all the flying horses in the world are my…You know what? Lets’ forget it.