On Earth, the dog is a subspecies of the wolf, and was one of the first animals to be domesticated by Humans. There were a wide variety of breeds of dogs, with different sizes, temperaments, and type of coats. Dogs were often kept as pets by Humans, and were also used for a variety of other tasks.
In Earth literature, Human playwright William Shakespeare had a character use a metaphor referring to soldiers as dogs, in his Julius Caesar, where the line went "Cry 'Havoc', and let slip the dogs of war". This quotation was memorably used by Klingon General Chang during his attack on the Federation starship USS Enterprise-A at Khitomer. (TOS movie, novelization & comic adaptation: The Undiscovered Country)
After being accidentally sent back in time from 2372 to July 1947, Constable Odo disguised himself as a German Shepherd while Quark, Rom and Nog were taken prisoner by the US Army Air Corps and held in the Roswell Army Air Field. In the form of the dog, he was able to eavesdrop on conversations and sneak around the base with impunity. He determined that Quark's Treasure was being stored in Hangar 18 of the facility. (DS9 episode: "Little Green Men")
By the 24th century, the above quotation came into use by a Federation reporter, referring to a band of genetically engineered mutant space pirates. By the mid-2370s, the Dogs of War had taken the name as their own. (NF novel: The Quiet Place)
- This quotation was also the source of the title for a number of Star Trek publications.
Jonathan Archer had a male beagle named Porthos that he took with him on board the Enterprise (NX-01). Porthos stayed with Archer for his entire time as captain of the Enterprise. (ENT episode: "Broken Bow", et al.)
As children, James T. Kirk and George Samuel Kirk, Jr. owned a Golden Labrador named Lady. When she became too old and ill, the boys begged to have her cloned, but their father George Samuel Kirk, Sr. refused, telling them they needed to respect the pet's life and death. (TOS novel: Prime Directive) Years later, in the 2270s and 2280s, Kirk owned another dog, a male Great Dane named Butler. (TNG movie & novelization: Generations)
During a Starfleet command officer cocktail party aboard Starbase 32 in 2333, Jean-Luc Picard overheard several guests discussing a sobering situation that one of them had found themselves in. Wishing to join the conversation, Picard asked if they were discussing a hostile force or undiscovered phenomenon, to which one of the guests replied that he was talking about putting his dog to sleep. (STA novel: Gauntlet)