Phobos (Φόβος, literally meaning "fear" in ancient Greek) is the god of fear and the son of Ares and Aphrodite. The suffix "-Phobia" comes from his name.

Deimos was the god (daemon) of terror and panic, and his twin-brother Phobos (Phobus) of fear, flight and battlefield rout. They were sons of the god Ares, who accompanied their father into battle, driving his chariot and spreading fear in their wake. As sons of Aphrodite, goddess of love, the twins represented fear of loss.

In classical art the two were usually represented as youths. Phobos was sometimes depicted with a lion or lion-like head.

Powers

•Phobos presumably has standard god powers.
•Phobos has the ability to make one see their worst fears, shown through eye contact.
•It is unable to be fought without gaining some sort of awareness; first the fish in the aquarium awaken Percy and Percy wakes Clarisse, even after this it seems to still take some amount of will power to fight back.
•Without his power he's a terrible fighter (likely never having to rely on it so he never trained).