Grass

“Um...is that thing tame?" Frank said.
The horse whinnied angrily.
"I don't think so," Percy guessed. "He just said, 'I will trample you to death, silly Chinese Canadian baby man'.”
― Rick Riordan, The Son of Neptune

“Poseidon,' said Chiron. 'Earthshaker, Stormbringer, Father of Horses. Hail, Perseus Jackson, Son of the Sea God.”
― Rick Riordan, The Lightning Thief

“How did you die?"
"We er....drowned in a bathtub."
"All three of you?"
"It was a big bathtub.”
― Rick Riordan, The Lightning Thief

“God alert!" Blackjack yelled. "It's the wine dude!
Mr. D sighed in exasperation. "The next person, or horse, who calls me the 'wine dude' will end up in a bottle of Merlot!”
― Rick Riordan, The Titan’s Curse