Mrs. O'Leary

Mrs. O'Leary is Percy's and formerly Deadulus' pet Hellhound. She is thought to be the only friendly Hellhound in existence.

Early History
Mrs. O'Leary was originally Quintus' (Daedalus') pet hellhound (now Percy's), and had been tamed by him. She is faithful to Quintus who met her by "many close calls with death and quite a few giant squeaky toys." She shows the same loyality to Percy

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 summon 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 Demigod Files
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 Charles Beckendorf (who died a few hours ago) would take care of her when Percy would be away for a quest or vacations. Pushing the sad thought away, Percy followed her until he found Nico 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.



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. Once the battle is over, Percy watches her play with Hannibal and says how she finally has found a play mate who's her size.

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.

Powers
Mrs. O'Leary can shadow travel, though it tires her. She is also rather strong and agile, and relatively intelligent. Though despite the fact that she is very intelligent she still acts like a regular dog sometimes (ie. near the beginning of the Battle of Manhattan she went over to a hotdog stand when the owner was asleep and Percy said she ate something that smelled like pepperoni). Before Thanatos came back to watch over souls again, Mrs. O'Leary was unable to die, being a monster and having the ability to reform, now she can die like any other hellhound.