What Hellhounds Reveal about Humans' Oldest Companion

Hellhounds, the fearsome canine beasts of the underworld who often serve as guardians to the very gates of hell, pop up in a wide variety of cultural folklore throughout history. They're also fairly commonplace in the stories we tell in modern times (think Cujo, Ghostbusters, The Omen). Why is that? What can these legendary hounds teach us about the history of humanity’s relationship with dogs.