The Book of Three (The Chronicles of Prydain #1) by Lloyd Alexander
Taran wanted to be a hero, and looking after a pig wasn’t exactly heroic, even though Hen Wen was an oracular pig. But the day that Hen Wen vanished, Taran was led into an enchanting and perilous world. With his band of followers, he confronted the Horned King and his terrible Cauldron-Born. These were the forces of evil, and only Hen Wen knew the secret of keeping the kingdom of Prydain safe from them. But who would find her first?