Agrius was a bear monster and the brother of Oreius; together they were called the "Bear Twins."
They were the illegitimate spawn of Polyphonte whom Aphrodite commanded must marry but chose not to, instead fleeing to Artemis. Artemis allowed Polyphonte to join the ranks of her Hunters of Artemis, but Aphrodite put her in a trance and made her fall in love with a bear. After falling in love with the bear, Artemis, in disgust, threw her out of the ranks of her Hunters by turning the animals against her. She fled back to her father where she gave birth to Agrius and Oreius. They were cannibals and disrespected humans and gods alike.
Zeus ordered Hermes to punish the two in whatever way the latter pleased. Hermes at first decided to dismember them, but Ares did not want this to happen to his grandsons. Instead, Hermes and Ares turned the twins into carnivorous birds where Agrius was turned into a vulture and Orius was turned into an eagle owl.
During the time of Percy Jackson's second year at Camp Half-Blood, the bear twins had found work with Luke Castellan who had aligned himself with Kronos. They traveled with him as his 'assistants', as he called them and followed him around the Princess Andromeda. When Percy Jackson, Annabeth Chase, and Tyson arrived unexpectedly, they first encountered these beasts and Luke told them their story.
While it is said that the Bear Twins like demigods for food, it is unknown if they have ever eaten an unlucky demigod. After Percy and his friends escape the ship and rescue Grover Underwood, they again encounter Agrius and Oreius. Luke duels with Percy while Agrius is in charge of holding Blackjack prisoner and Oreius is commanded to feast upon Annabeth and Grover. The timely arrival of Chiron and the Party Ponies startle the monsters, which allows Blackjack to slip out of Agrius' grip and fatally kick him in the head. Agrius travels to Tartarus just moments after his brother.
It's revealed that Blackjack credits Percy with his escape from Luke and Agrius and as such has become his loyal friend and steed.
Agrius was obviously the more intelligent (or at least more commanding) of the twins, as he ordered his noticeably less intelligent brother to do various tasks including 'punishing himself' which entailed Oreius slamming his own head into an object or causing other bodily harm. He also scolded his brother once and made him whimper.
Both were described as being eight feet tall, wearing nothing but blue jeans, being covered in shag-carpeted thick brown fur, with claws for fingernails, feet like paws, snouts for noses, and all their teeth being pointed canines.
- Agrius was meant to be in the Percy Jackson: Sea of Monsters movie, but was cut out of it before it was released and was replaced with a Manticore. He was going to be animatronic.