Dog, A Man's Best Friend

Previous to the 19th century, dogs, other than lap dogs, were largely functional. Used for activities such as hunting, watching and guarding, language describing the dog often reflected these positions within society. According to the Oxford English Dictionary, “In the oldest proverbs and phrases dogs are rarely depicted as faithful or as man’s best friend, but as vicious, ravening, or watchful.” Beginning in the 18th century, multiplying in the 19th and flourishing in the 20th century, language and attitudes towards dogs began to shift. Possibly, this societal shift can be attributed to discovery of the rabies vaccine in 1869.
The earliest citation of the actual word choice - in English - is traced to a poem printed in the The New-York Literary Journal, Volume 4, 1821:
The faithful dog - why should I strive