"They were two sizable utukku, stolidly marching"

Statue of the Assyrian demon Pazuzu.
Creative Commons Attribution Share AlikeStatue of the Assyrian demon Pazuzu. - Credit: PHGCOM
According to Bartimaeus, the Utukku are type of djinni known for its unintelligent devotion to violence; they were favoured by the Assyrians. Xerxes (aka Eagle-beak) appearance might be based upon some Assyrian demon like Pazuzu, for example, who is often depicted as a combination of diverse animal and human parts.