Hellhound

A hellhound is a massive, dog-like monster from the Underworld. Hellhounds are feared by half-bloods (except children of Hades, though there are some exceptions, like Mrs. O'Leary). Hellhounds are very large and can shadow travel. Percy first experienced shadow traveling, done by Nico, when he and Mrs.O'Leary went to Westport, Connecticut, May Castellan's hometown. It takes a lot of energy for hellhounds to shadow travel, especially the first couple of times they use it, and they usually must rest afterwards. Most hellhounds can shadow travel, as shown in The Last Olympian. Through the Mist Mrs. O'Leary was seen as a poodle (The Demigod Files).

History
Hellhounds were sons and daughters of Nyx (mother) and Cerberus (father).

Appearance
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 large as a garbage truck. (e.g. Mrs. O'Leary)

Powers

 * They can shadow travel.
 * Very fast
 * Very strong

The Lightning Thief
Just after Percy's first capture the flag game, he had been attacked and almost killed by a hellhound, the hellhound was killed by arrows fired by Chiron. (This is when Percy is claimed by Poseidon when he steps into the river and is healed) Later it was revealed that it was Luke Castellan who was working for Kronos had summoned it, since no monster (unless given permission) can enter within the camp's borders.

The Battle of the Labyrinth
The main hellhound that appeared was Mrs. O'Leary, the only friendly hellhound. She was probably trained by Quintus (Daedalus), because she is very loyal to him and was very sad when he died. Her bark is said to be "slightly louder than an artillery gun." Quintus claimed that Mrs. O'Leary met him after "many close calls with death and quite a few giant chew toys".

The Last Olympian
Again Mrs. O'Leary appeared in this book. She used shadow travel for the first time in the book, after Nico told Percy how to do it. During the final battle, hellhounds were a part of Kronos's army. They mainly shadow traveled behind the demigods' lines.

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.

Magic Items

 * Stygian Ice Whistle: It is unsure if it can be used to call any hellhound, but when Percy 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).