“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.”
Blackjack, The Mark of Athena.