from what ive researched, if you have noticed a hell hound, which has noticed you, then you should never speak about it to anyone for a year and a half, or face death the next day. A Hell Hound is a supernatural dog, found in mythology, folklore and fiction. A wide variety of ominous or hellish supernatural dogs occur in mythologies around the world, similar to the ubiquitous dragon. Features that have been attributed to hellhounds include black fur, glowing red or sometimes glowing yellow eyes, super strength or speed, ghostly or phantom characteristics, foul odor, and sometimes even the ability to talk. In cultures that associates the afterlife with fire, hellhounds may have fire-based abilities and appearance. They are often assigned to guard the entrances to the World of The Dead, such as graveyards and burial grounds, or undertake other duties related to the afterlife or the supernatural, such as hunting down lost souls or guarding a supernatural treasure. In European legends, seeing a hellhound or hearing it howl may be either an omen or a cause of imminent death.
Some supernatural dogs, such as the Welsh Cwn Annwn, were actually believed to be benign. However, seeing them was still considered an omen of death. But of course this is all folklore, I for one would never wish to see a black demon dog.