Hell hounds are attracted to a certain energy, generally death. They are messengers and scouts of the psychopomps.
Certain people are more inclined to being naturally aligned with such energy. Usually they are a tad morbid ("I want to grow up to be a mortician! Yay!") and choose deities that have a say in death and war. They are commonly very quiet about their practise because it isn't very accepted. Most "dark" or "black" magick practitioners aren't selfish and harmful but are mislabeled due to the company they choose to keep, that of the dead. They are actually people that resonate with below as apposed to above, the Underworld over the Upperworld. I'm one of those people. Such people generally act as psychopomps themselves, helping and honoring deceased souls, our ancestors, our loved ones, our dead.
Hell hounds are black dogs, commonly with red eyes. They are messengers portending disaster or death to either the person that sees them or the ones directly associated to them. They are an underworld creature. I never met one in person because my Lady sends black birds. Some are inclined to dogs, such as with Anubis and Hades.