Perhaps the two most important and prominent qualities which dogs have symbolised in literature and myth down the ages are vigilance and loyalty. However, there are also some curious and lesser-known aspects of dog-symbolism which are worth probing; we’ll get to these in time. As the vast and informative The Penguin Dictionary of Symbols (Penguin dictionaries) notes, the symbolism of the dog in different cultures is ‘extremely complex’, with many religions and myths linking the dog to death, hell, and the Underworld.
Let’s start with vigilance first.
Dogs are often associated with vigilance, with the term ‘watchdog’ having come to mean someone or something (such as an official investigative body) which watches over something and ensures its standards are maintained. This idea of dogs as ‘on watch’ is an old symbol which goes back several millennia, to classical myth.
Dog symbolism in mythology
This idea of dogs as watchful and vigilant goes back to classical myth: the most famous example is Cerberus, the three-headed dog that guarded the entrance to the Underworld in Greek mythology. Hecate, the Greek goddess of witchcraft, was said to be accompanied by a pack of fighting dogs or hell-hounds. Indeed, in some depictions of her, Hecate, like Cerberus, had three heads, one of which was a dog’s head (the other two, in case you’re interested, were a snake’s head and a horse’s).
In Norse and Germanic myth, dogs take on an even more sinister turn. Garm is a hell-hound which, during Ragnarök or the last battle, kills the god Tyr (and, in turn, is killed by him) in Norse mythology.
In ancient Egypt, one of their gods, Anubis, had the dead of a jackal. Anubis was a god of the dead, so once again, dogs have some divine association with death and the afterlife. Dogs often symbolise the journey from the land of the living to the abode of the dead. But in ancient Egyptian iconography, a number of dog-headed deities guarded holy sites, not just the underworld. The role of ‘guard dog’ is a useful one for both gods and mortals, it seems.
Dogs as symbols of loyalty
Dogs are also symbols of, and are associated with, loyalty: in the Middle Ages they were symbols of feudal loyalty or marital fidelity, as Hans Biedermann notes in his excellent The Wordsworth Dictionary of Symbolism: Cultural Icons and the Meanings Behind Them (Wordsworth Reference). A common name for pet dogs is Fido, which literally means ‘faithful’: this comes from a real Italian dog, famous 
for its unwavering loyalty to its dead master in the 1940s. Perhaps the most famous novel ever written about a dog, the novel Greyfriars Bobby, embodies and symbolises this same idea.
Few people know much more about Greyfriars Bobby than the name, the fact that it’s a Scottish tale, and that it embodies this notion of faithfulness. Its author, Eleanor Atkinson, is hardly known about now. The novel was published in 1912, and is unusual in that it is written from the perspective of the Skye terrier which gives the novel its title. Auld Jock, Bobby’s owner, has a close bond with his pet terrier, with the two of them helping out each other. When (spoiler alert) Jock dies, Bobby refuses to leave his master’s side, even when Jock is buried. Bobby ends up guarding Jock’s grave, by day and night, thus neatly symbolising the two main features associated with dogs: fidelity and vigilance.
The real Greyfriars Bobby, a Skye terrier who lived in the nineteenth century, was just as remarkable as the fictionalised version, spending 14 years guarding the grave of his dead master, a nightwatchman (fittingly) named John Gray.
This story of a loyal dog remaining true to its master even after that master’s death is found elsewhere in the literary world: Emily Brontë’s dog, Keeper, followed her coffin to the grave when Brontë died in 1848 and, for weeks after, moaned and howled outside her bedroom door.
Dogs in poetry
Many poets have paid tribute to the dog – often their own dogs. Thomas Hardy’s ‘A Popular Personage at Home’ is one of two poems Hardy (1840-1928) wrote about his beloved dog of 13 years, Wessex, who died in 1926, two years before Hardy himself.
However, what makes ‘A Popular Personage at Home’ especially notable is that Hardy wrote the poem from the perspective of the dog, allowing ‘Wessex’ to speak for himself. But the speaker is reminiscent of Hardy himself, with some suggestive nods to the changes wrought upon the English landscape (since the Industrial Revolution) and Hardy’s well-documented fear that the England he knew and loved was not going to last, and had indeed already begun to fade from view:
‘No doubt I shall always cross this sill,
And turn the corner, and stand steady,
Gazing back for my Mistress till
She reaches where I have run already,
‘And that this meadow with its brook,
And bulrush, even as it appears
As I plunge by with hasty look,
Will stay the same a thousand years.’
Wessex is a symbol of the enduring landscape: Wessex the dog symbolises Wessex the land.
In ‘The Power of the Dog’, Rudyard Kipling (1865-1936) extolled the dog’s most famous virtue – its undying loyalty and devotion to its owner – but also warns against giving your heart to a dog for it ‘to tear’. Such is ‘the power of the dog’:
From men and women to fill our day;
And when we are certain of sorrow in store,
Why do we always arrange for more?
Brothers and sisters, I bid you beware
Of giving your heart to a dog to tear.