The Titans Curse by Rick Riordan

The Titan's Curse is a 2007 fantasy-adventure novel based on Greek mythology written by Rick Riordan. The Titan's Curse is the third novel in the Percy Jackson & the Olympians series and the sequel to The Sea of Monsters. It charts the adventures of the fourteen-year-old demigod Percy Jackson as he and his friends go on a dangerous quest to rescue his friend Annabeth Chase and the Greek goddess Artemis, who have both been kidnapped.
Percy Jackson, Annabeth Chase and Thalia Grace, go to Westover Hall in Maine after Grover Underwood discovers two sibling half-bloods named Nico and Bianca di Angelo. They narrowly escape an attack by a manticore named "Dr. Thorn," with some help from Artemis and her hunters, which include Zoë Nightshade. Though they survive the attack, Annabeth is captured by Dr. Thorn. Percy and his friends must look for Artemis before the winter solstice meeting of the Olympian council, when the goddess's influence could change an important vote on the war with the Titans. Percy joins Thalia, Zoe, Bianca di Angelo, and Grover on their dangerous quest. They also are searching for a rare monster that Artemis was trying to hunt down when she went missing; it is so strong that it can obliterate Olympus. Percy is called by a hippocampus to rescue a marine cow-like creature trapped in the Long Island Sound called the Ophiotaurus, whom Percy names Bessie, not knowing it is male ... (Summary by Wikipedia)