Hellhound

A hellhound is a massive dog like the monster from the Fields of Punishment and is a servant of Hades. Hellhounds are feared by half-bloods, though there are some exceptions, like Mrs. O'Leary. Hellhounds are very large and can shadow travel. Percy first experienced shadow travelling 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

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. They are about the size of a small bear, but can be as large as a garbage truck. (e.g. Mrs. O'Leary)

Powers

 * Can shadow travel
 * Fast
 * Strong

The monster boys
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. Later it was revealed that it was Luke Castellan who 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 demi-gods' lines.

Magic Items

 * Stygian Ice whistle: It is unsure if it calls any hellhound, but when Percy blew 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

Percy Jackson and the Olympians: The Lightning Thief
In the Percy Jackson film, two hellhounds are about to attack Percy, Annabeth, and Grover when they enter Hades' Palace but Persephone stops them from hurting them. They appear notably different than the official artwork for the books, being smaller than a human and with much less hair. They also appear more demonic compared to the dog-like appearence in the book series.