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Mrs. O'Leary, the only friendly Hellhound who is able to transform into a dog

Hellhounds are massive, dog-like Monsters from the Underworld.


Hellhounds are children of the goddess Nyx and Cerberus, the guard dog of the Underworld. They are feared by demigods save for children of Hades and other chthonic deities.

​​​​​​Percy Jackson and the Olympians

The Lightning Thief

Just after Percy's first capture the flag game, he is attacked and almost killed by a hellhound, which is defeated by arrows fired by Chiron. It is later revealed that Luke Castellan, who was working for Kronos at the time, had summoned it since no monster can enter within the camp's borders unless given permission.

The Sea of Monsters

Two hellhounds were seen on the Princess Andromeda.

The Battle of the Labyrinth

The main hellhound that appears is Mrs. O'Leary, the only friendly hellhound. She was probably trained by Quintus, because she is very loyal to him and is very sad when he dies. Her bark is said to be "slightly louder than an artillery gun." Quintus claims that he and Mrs. O'Leary became friends through "many close calls with death and quite a few giant chew toys".

The Demigod Files

Through the Mist, Mrs. O'Leary was seen as a poodle.

The Last Olympian

Mrs. O'Leary appears when Percy shadow travels for the first time after Nico teaches him how to do it. During the final battle, numerous hellhounds are a part of Kronos' army. They mainly shadow travel behind the demigods' lines.

Pandava Quintet

AruShah.png Not part of Riordanverse

The following article/section is from the Pandava Quintet continuity under Rick Riordan Presents and not the Riordanverse canon.

Aru Shah and the End of Time

Ek and Do are two hellhounds who guard the Kingdom of Death.

Storm Runner Trilogy

Zane Obispo.jpg Not part of Riordanverse

The following article/section is from the Storm Runner/Shadow Bruja continuity under Rick Riordan Presents and not the Riordanverse canon.

The Storm Runner

Rosie, now a hellhound

Two hellhounds appear next to Ixtab when she arrives at the Old World, one of them is Rosie after she was killed by a Demon Runner.


Hellhounds are usually described as being large, black mastiffs with glowing red eyes. The average size of hellhounds are about the size of a grizzly bear or a rhino, but can be as tall and large as a garbage truck (e.g. Mrs. O'Leary).


  • Shadow Travel: They can travel through shadows. However, it takes a lot of energy, especially the first couple of times they use it, and they usually must rest afterwards.
  • Enhanced Physical Prowess: They have superhuman strength and speed.

Magic Items

  • Stygian Ice Whistle: It is unsure if it can be used to call any hellhound, but when the user blew into it, only Mrs. O'Leary came. This might have been due to Kronos preventing them from answering the call or it might be specifically made for a certain hellhound (most likely the latter).


Percy Jackson and the Olympians: The Lightning Thief

Hellhounds as they appear in The Lightning Thief film

In the film, Percy does not encounter a hellhound in the forest, but is instead visited by Hades at the bonfire. Later in the Underworld when they enter his palace, they are 'greeted' by angry hellhounds, but Persephone scolds them and they leave. She tells the demigods that the hellhounds sense the presence of another animal, indicating Grover, a satyr. In the film, they resemble gargoyles, having a grayish lion like face and fur.


  • The film version made hellhounds smaller, but more ferocious, having flat heads and with larger, reptilian-like teeth.
  • In real life, Hellhounds come from English/British folklore and have been sighted throughout Great Britain
  • Notable names for these beasts include Black Shuck, Barghest, Gwyllgi, Cu Sith, Gytrash and Yeth Hound.
  • Hellhounds were made famous in the Sherlock Holmes Novel, The Hound of The Baskervilles by Sir Arthur Conan Doyle


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Friendly Monsters: Argus | Blackjack | Briares | Bombilo | Chiron | Cottus | Don | Ella | Festus | Gleeson Hedge | Gray | Grover Underwood | Guido | Gyges | Lysas | Mellie | Mrs. O'Leary | Ophiotaurus | Peaches | Peleus | Porkpie | Rainbow | Scipio | Tyson | Tempest | Sssssarah
Enemy Monsters: Antaeus | Agrius and Oreius | Arachne | Cacus | Carthaginian Serpent | Charybdis and Scylla | Chimera | Chrysaor | Clazmonian Sow | Colchis Bulls | Echidna | Euryale | Geryon | Joe Bob | Kampê | Kekrops | Ladon | Lamia | Manticore | Medusa | Minotaur | Nanette | Nemean Lion | Polyphemus | Python | Sphinx | Stheno | Typhon | Trojan Sea Monster
Neutral Monsters: Carnivorous Sheep | Cerberus | Erymanthian Boar | Gray Sisters | Flesh-Eating Horse | Furies | Gryphon | Orthus | Skolopendra | Sybaris