Argentum

Argentum is one of Reyna's automaton guard dogs, along with Aurum. Argentum is the silver hound, as his name means 'silver' in Latin, and he 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 Heroes of Olympus

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. Reyna warns him that they will attack if she wishes it, or if he steals anything. She also warns that they can tell when people don't tell the truth and that they hate liars.

The Mark of Athena

Aurum and Argentum appear with Reyna when she talks with Annabeth near the beginning of the book, and also when Reyna confronts Annabeth later in the book, at Charleston.

The Blood of Olympus

Argentum appears while protecting Reyna, Nico and Coach Hedge. While fighting ghosts in Pompeii, his head gets twisted 45 degrees and he loses an eye. However, the Amazons repaired him.

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, as he is allowed to enter New Rome, but as Annabeth noticed, Argentum and Aurum could easily tear her apart with their sharp metal teeth.

Items
Personal Weapons:	Riptide \| Annabeth's Knife \| Backbiter \| Master Bolt \| Poseidon's Trident \| Nico's Sword \| Thalia's Spear \| Aegis \| Ares' Sword \| Ares' Shield \| Caduceus \| Apollo's Golden Bow \| Silver Bow \| Artemis' Knives \| Hades' Sword \| Hades' Staff \| Kronos' Scythe \| Ivlivs \| Katoptris \| Hazel's Spatha \| Frank's Spear \| Reyna's Spear \| Juno's Gladius \| Annabeth's Sword \| Sumarbrander \| Gungnir \| Gjallar \| Mjolnir \| Gríðarvölr \| Khopesh \| Mallory's Serrated Knives \| Meg's Twin Imperial Gold siccae blades \| Thyrsus \| Hecate's Torches \| Minotaur's Axe \| Leroy's Sword \| Tyson's Javelin \| Alex's Garrote Wire \| Piper's Sword
Magical Items:	Annabeth's Yankees Cap \| Helm of Darkness \| Pandora's Pithos \| Winged Shoes \| Golden Fleece \| Flying Chariot \| Golden Apple \| Apples of Immortality \| Greek Fire \| Hermes' Multivitamins \| Nectar and Ambrosia \| Stygian Ice Whistle \| Keys of Hades \| Leo's Magical Toolbelt \| Serapis' Staff \| Magic 8 Ball \| Arrow of Dodona \| Pig Ball \| Mechanical Spider \| Angel Statues \| Athena Parthenos \| Chiron's Wheelchair \| Diocletian's Scepter \| Flaming Dodgeball \| Gleipnir \| Poseidon's Pearls \| Queen Hippolyta's Belt \| Mistletoe Arrow \| Frank's Stick \| Expand-o-Duck \| Caligula's Caligae \| Ran's Net \| Rune Stones \| Nábrók \| Pomegranate
Spoils of War:	Minotaur's Horn \| Medusa's Head \| Kampê's Scimitars \| Nemean Lion's Pelt \| Gorgon Blood \| Cornucopia \| Lydian Drakon Hide \| Phineas' Robe and Slippers \| Triptolemus's Almanac \| Odysseus' Astrolabe
Items:	Camp Necklace \| Chameleon Armor \| Daedalus' Laptop \| Golden drachma \| Denarius \| Red Gold \| Mark of Athena \| Thalia's Shield \| Video Shield \| Wristwatch Shield \| Golden Mango \| Sibylline Books \| Letter of Recommendation
Ships	Amos' Boat \| Argo II \| CSS Birmingham \| Egyptian Queen \| Julia Drusilla Yachts \| Pax \| Queen Anne's Revenge \| Reed Boat \| Naglfar \| Gorgyra's Canoe
Blessed Metals:	Adamantine \| Celestial Bronze \| Imperial Gold \| Stygian Iron \| Bone Steel

The Heroes of Olympus
Core Series:	The Lost Hero \| The Son of Neptune \| The Mark of Athena \| The House of Hades \| The Blood of Olympus
Main Characters:	Jason Grace \| Piper McLean \| Leo Valdez \| Percy Jackson \| Frank Zhang \| Hazel Levesque \| Annabeth Chase \| Iapetus/Bob \| Reyna Ramírez-Arellano \| Nico di Angelo \| Gleeson Hedge
Secondary Characters:	Hylla Ramírez-Arellano \| Dakota \| Tyson \| Ella \| Octavian \| Halcyon Green \| Dr. Howard Claymore \| Alabaster C. Torrington \| Lamia
Minor Characters:	Rachel Elizabeth Dare \| Grover Underwood \| Thalia Grace \| Clarisse La Rue \| Fleecy \| Mrs. O'Leary \| Kinzie \| Arion \| Calypso \| Lou Ellen Blackstone \| Chiron \| Will Solace \| Tristan McLean \| Don \| Julia \| Jacob \| Michael Varus \| Burly Black \| Medea \| Midas \| Lityerses \| Phineas \| Otrera \| Echo \| Narcissus \| Sciron \| Pasiphaë \| Lycaon
Olympian Gods:	Zeus \| Hera \| Poseidon \| Hades \| Ares \| Demeter \| Athena \| Apollo \| Artemis \| Hephaestus \| Aphrodite \| Hermes \| Dionysus
Minor Gods:	Achelous \| Aeolus \| Asclepius \| Boreas \| Eurus \| Hecate \| Iris \| Hypnos \| Keto \| Khione \| Kymopoleia \| Mithras \| Nemesis \| Nike \| Notus \| Phorcys \| Serapis \| Thanatos \| Triptolemus \| Zephyros
Roman Gods:	Jupiter \| Juno \| Neptune \| Pluto \| Mars \| Minerva \| Ceres \| Lupa \| Bellona \| Fortuna \| Janus \| Terminus \| Vulcan \| Mercury \| Apollo (Roman) \| Diana \| Venus \| Bacchus \| Pomona \| Aquilon \| Hercules \| Cupid \| Auster \| Favonius \| Letus \| Victoria
Giants:	Enceladus \| Porphyrion \| Alcyoneus \| Polybotes \| Ephialtes \| Otis \| Damasen \| Clytius \| Mimas \| Orion \| Hippolytos \| Thoon \| Periboia
Undead:	Gray \| Zombie
Primordial Gods:	Gaea \| Tartarus \| Ourae \| Nyx \| Chaos \| Ouranos \| Akhlys \| Hemera \| Elpis \| Spes
Monsters and Magical Creatures:	Cynocephali \| Gorgon \| Gryphon \| Harpy \| Basilisk \| Lycanthrope \| Gegeines \| Cyclops \| Katobleps \| Unicorn \| Giant Eagle \| Ichthyocentaur \| Satyr/Faun \| Storm Spirit \| Laistrygonian Giant \| Lares \| Mania
Related Content:	Rick Riordan \| Haley Riordan \| Percy Jackson and the Olympians \| Percy Jackson and the Olympians: The Ultimate Guide \| The Demigod Files \| The Demigod Diaries \| The Son of Sobek \| The Singer of Apollo \| The Staff of Serapis \| Percy Jackson's Greek Gods \| Percy Jackson's Greek Heroes \| The Crown of Ptolemy \| Demigods & Magicians \| Demigods of Olympus \| Percy Jackson Demigod Collection