Mrs. O'Leary

Mrs. O'Leary is Percy's pet hellhound. She is thought to be the only friendly hellhound in existence. She is unnaturally large, even for a hellhound.

History
Mrs. O'Leary was originally Quintus' (Daedalus') pet hellhound. Quintus tamed her and they became very close. She is faithful to Quintus, then to Percy. According to Quintus, the story of how he got her involves "many close calls with death and quite a few giant squeaky toys."

The Battle of the Labyrinth
Mrs. O'Leary came to Camp Half-Blood with her owner Quintus. When Percy first saw her, he attacked her but was stopped by Quintus who wouldn't let anything happen to his pet. Mrs. O'Leary turned out not to be violent, but very sweet and friendly, making friends with Percy and Tyson, and often playing with a shield as a Frisbee and a combat dummy to play fetch. When Percy was about to journey into the labyrinth, Quintus gave him a whistle made of Stygian Ice so if Percy ever needed help, he could use it to call Mrs. O'Leary.

In Antaeus' Arena, Percy used the whistle to call Mrs. O'Leary for help to save himself Annabeth Chase, Rachel Elizabeth Dare and Ethan Nakamura, but she disappeared after they fled from Luke Castellan's minions. Mrs. O'Leary found her way back to Quintus who was revealed to be Daedalus who had originally built the labyrinth. When they were attacked by Luke's monsters, Percy and his friends fled, but Quintus wouldn't leave his pet. They made it out and joined the Battle of the Labyrinth at camp. With the help of Nico di Angelo, son of Hades, Daedalus passed on and left Mrs. O'Leary in Percy's care.

When Percy left camp, he left Mrs. O'Leary in the care of Charles Beckendorf who forged her a collar, a toy and other useful items.

The Sword of Hades
In The Sword of Hades, Mrs. O'Leary arrived in the middle of Percy's English exams and The Mist made everyone believe she was a poodle, which impressed Percy due to her size. He followed her into the park where he came across Thalia Grace and Nico di Angelo who, along with Percy, were all taken to the Underworld. Mrs. O'Leary had tried to warn him, but she was too late.

The Last Olympian
Percy was ambushed by a gleeful Mrs. O'Leary who was happy to see him at camp. But he was also sad, as he remembered how Charels Beckendorf (who died a few hours ago on the Princess Andromeda) would take care of her when Percy would be away for a quest or vacations. Beckendorf had also welded Mrs. O'Leary's favorite bronze chewing bone, and her collar. Pushing the sad thought away, Percy followed her until he found Nico di Angelo with Juniper and Leneus. Nico took the hellhound for a walk while the others talked, as Juniper and Leneus were both afraid of dogs. Nico got Percy to leave with him afterwards to Luke's mother's house. Percy rode on Mrs. O'Leary who used shadow travel which tired her. After the meeting, they met Hestia who gave Mrs. O'Leary a large dog treat. Shadow travel greatly tired Mrs. O'Leary, as it also did to Nico. The huge hellhound let out a yawn before she turned in a circle and flopped down so hard the ground shook.

With the help of Grover, they were able to get to the Underworld where Mrs. O'Leary made friends with Cerberus. Percy called her off when the Furies came to collect them. After Percy and Nico escaped, she transported them to the River Styx where Percy bathed to receive the curse of Achilles. Percy fought Hades' soldiers to protect Nico and his pet. They left the Underworld and met the other Demigods by the Empire State Building.

Mrs. O'Leary took part in the Second Olympian War, but got injured fighting the Drakon. She got a message from Percy to Nico for reinforcements and she took an injured Chiron back to camp.

During the war, Percy had to face a hellhound, but hesitated to kill it. He reminded himself: "This is not Mrs. O'Leary. This is an untamed monster. It will kill me and all my friends."

The Son of Neptune
In one of his dreams, Percy sees Tyson riding her as they look for him. Percy then whistles for her when he and his friends are right near Camp Jupiter and she helps them haul tons of Imperial Gold and fights in the battle, and attack a Gorgon. Once the battle is over, Percy watches her play with Hannibal and says how she has finally found a play mate who she can rough-house with.

Appearance
Mrs. O'Leary is very large, very scary and described as a mastiff. Percy describes her as "bigger than a tank and barks slightly louder than an artillery gun". Mortals, on at least one occasion, see her as a large standard poodle, rather than an impossibly big dog. That fact truly shows how powerful the Mist is.

Abilities

 * Shadow Travel - Like all hellhounds, Mrs. O'Leary can shadow travel, though this is shown to tire her.
 * Immortality - Like all monsters, Mrs. O'Leary cannot die. She will just be reborn from Tartarus, like any other monster, unless killed by Stygian Iron. Whether her personality and friendliness would be affected by this is unknown.

Trivia

 * When Daedalus is asked how he got a hellhound for a pet, he told Percy it was a long story that involved a lot of close calls with death and quite a few giant squeaky toys. When Hazel Levesque questioned Percy about having a hellhound for a pet, Percy told her it was a long story just like Daedalus told him. Percy's story also involved a lot of close calls with death and a few giant chew toys.
 * Mrs. O'Leary the hellhound is likely named after Catherine O'Leary, a woman from the 19th century who, in popular culture, is fallaciously believed to be the owner of a cow that kicked over a lantern, causing the Great Chicago Fire of 1871. Catherine is affectionately named "Mrs. O'Leary" in popular culture.