Cerberus

Cerberus is a red and black three headed dog who guards the entrance to the Place of Eternal Darkness.

Additional Info
Cerberus is the guard of the entrance to the Place of Eternal Darkness. He can control fire with his paws, and will attack anyone he sees except for Achlys. As Achlys said, he can eat a dead cats soul, and when a dead cat's soul is gone, it will not be able to enter the Place of Eternal Darkness or the Place of Starry Skies. Also, a spirit who's soul is gone will be unable to speak, think, and be seen by any cat whatsoever. The cat spirit will suffer in silence for all of eternity. How fun.

When the group travels to the Place of Eternal Darkness, they are attacked by the Cerberus. Orpheus, by accident brushes his tail up against something in a trash can. It made a sound that the Cerberus likes, so he stopped attacking. The two became friends before Orpheus' death shortly after.

Orpheus can now call the Cerberus to his aid whenever he wants, though he rarely needs to do this and it would be more of a matter of want for him.

Trivia

 * In Greek and Roman mythology, the Cerberus is the guard of Hades.
 * The only way to get passed him is to make him sleep with music.