Argentum

Argentum is one of Reyna's automaton guard dogs, along with Aurum. Argentum is the silver hound, as its name means silver in Latin, and has eyes made of rubies. Both of the dogs have the ability to know when a person is lying and will then attack the person.

The Son of Neptune
While not having a large role in the book, Argentum and Aurum follow Reyna both times she talks to Percy alone. They seem to dislike Percy Jackson greatly, growling at him almost the entire time they are in the same room as him. She warns him that they can tell when people don't tell the truth and that they hate liars.

Trivia

 * Argentum is the only automaton in both series to be made of silver.
 * Argentum and its counterpart, Aurum, may have been modeled after King Alcinous's gold and silver guard dogs, mentioned in Book Eight of The Odyssey.
 * Argentum is not considered a weapon but as Annabeth noticed Arum and him could easily tear her apart wit their sharp metal teeth.