Today's question was sent in by a fan, and reads: Could you maybe talk about the connection between dogs and the underworld that appears in so many cultures' mythologies? And why yes, we can!
Dogs are really interesting creatures, sociologically speaking, which means that they're also very interesting mythologically. They're not a naturally occurring phenomenon but a human-made one; humans intentionally domesticated and selectively bred them for their own purposes, creating separate species of creatures that have things in common with their wild forbears but are no longer the same. The dog is an animal but one of very few animals that is of humanity instead of the wilderness. It belongs to us, in a cultural sense, and has for millennium upon millenium, and that gives many mythologies a peculiarly specific attitude toward the dog and what it means in a symbolic sense. (Incidentally, domesticated cats are also mythologically special for many of the same reasons, but let's let the canines have this one.)
Dogs crop up related to the ideas of death, the afterlife and the underworld across several different mythologies, usually because of their association with guardianship. One of the fundamental reasons for humans to domesticate dogs was so that they could serve as guards, warning system and defense force against hostile predators or other people, and they're therefore perfect fits for protecting fragile life from the terrors of what has gone beyond it. Dogs are often liminal creatures - animals that inhabit the borderlines between places and people, the better to protect and guard them, so they appear on the border between life and death in many cultures and across many times.
The most famous underworld dog for most western audiences is of course Cerberus, the three-headed hound of Hades that guards the entrance and exit to the underworld and allows none to pass. The dog functions as a guardian who keeps the lands of living and dead necessarily separate; he refuses to allow the living to pass into the land of the dead, where they don't belong, or to let the dead back out to harass the land of the living. As one of the monstrous children of Echidna and Typhon, he is clearly a dangerous beast that no one is particularly happy about encountering, but it's his very nature as a dog that allows him to be a useful tool of the gods rather than a monster that needs to be put down.
Cerberus isn't the only canine connection the underworld in classical mythology, however; the goddess Hecate, lady of the crossroads and the lines between life and death herself, is also highly associated with the dog. As one of the underworld goddesses herself, she is accompanied by dogs in artwork and heralded by their howling, and her cults worshiped her by keeping dogs in her temples and offering them to her as sacrifices. One of her titles is Skylakagetis, meaning literally "Leader of the Dogs", and she both appears in ancient myth as a dog herself, playing with young Persephone in the form of a loyal companion, and transforms queen Hecabe into a dog to become her own follower after the mortal woman was killed. Later, the Romans (who called her Trivia, meaning "three-part") even associated her with the dog star, which as the eye of the constellation Canis must of course belong to her.
Hecate is, like the dogs themselves, a goddess of the space between life and death, controlling the flow of each so that they do not interfere with one another, and so it makes perfect sense that the dog is her most important totem animal.
But the Greco-Roman world is just an easy starting place - dogs are all over the landscape when it comes to the underworld and the gods who administer it. Farther east, the Hindu underworld Naraka is guarded by Shyama and Sabala, two four-eyed dogs faithful to the death god Yama, who prevent unauthorized entrance or exit from the realms of death. The dogs not only guard the underworld but also act as messengers for Yama himself, further extending their role as inhabitors of the space between life and death, and guard the dead who are on their way to Naraka to ensure that they arrive safe.
Persian mythology, which shares common roots with Hinduism, also mentions four-eyed dogs, guarding the bridge of Chinvat that the dead must cross to achieve heaven or damnation. Traditional Zoroastrian funeral rites involve bringing a four-eyed dog (i.e., a dog with markings on its head that make it look four-eyed) in to gaze upon the dead person in a ritual called sagdid, in order that it truly confirm the person's death and allow them to carry on toward the afterlife, again confirming the idea of the dog's importance when crossing the threshold between life and death.
Stroll back up to northern Europe, and the guardian dog of the underworld is present again Norse mythology, this time as the fearsome hound Garm that guards the gates of Helheim. Bloodstained and fearsome, Garm howls to signify the beginning of Ragnarok, symbolically both heralding the imminent death of everyone and everything and performing his function as a watchdog who warns of danger, and once the apocalypse begins he is foretold to find and destroy the god Tyr. It's especially noteworthy that Garm is a dog instead of a wolf, when wolves of various dangerous qualities are so widespread in Norse mythology, and that he is the destined killer of Tyr, the only god to have tangled with the greatest among wolves and survived.
And then we can jump across the ocean completely to Mexico, where Xolotl, the god of the path between death and life and bringer of souls to the underworld, is depicted as a great dog-headed deity, skeletal and frightening but also undeniably canine. Ancient Mexican death rituals involved dogs heavily, believing that as the guardians of mankind they extended that protection into the afterlife; it was believed that the dead had to cross many perils and trials to travel to the underworld, but that each of them was granted a ghostly dog to help, guide and protect them on their way. Often, a dog might be killed when an important person died to ensure that they had a companion for the hard road ahead or a faithful dog buried along with its owner so they could stay together, and in some areas dogs were specifically raised for this purpose, bred from birth to cross over into death with their masters.
That's just scratching the tip of the death-and-dog iceberg, really; there are tons more cultures that associated dogs with the underworld or the boundary between life and death, and this post is already super long. Even Christianity has them, with its popular image of the hellhound that torments and harries sinners. And the guardian dog who watches the threshold isn't the only kind of death-aligned canine - other cultures connect the dog to hunting and say that they accompany the gods of death when they go out seeking mortal souls, baying for unwary humans whose time has come.
It's tempting to write dogs off as less important than the dangerous wild beasts that also populate mythology, but to do so would be a mistake. In spite of - or, in many cases, because of - their close connection to humanity, dogs are among the most powerful and personal of mythological totem creatures.