Dogs were companions to humans long before written history, but being a guardian and a hunting partner is quite different to being a pet. In the Middle Ages they were not usually pets as they are today.
The majority of medieval dogs had to work for a living and their most common vocation was as guard dogs either of homes or of goods and livestock. In this capacity dogs were found at all levels of society. Hunting dogs were also important, especially in aristocratic culture and they feature prominently in the sources left to us.
Unlike the mongrel guard dogs of merchants and shepherds, the practice of dog breeding (maybe of Roman origin) survived in the dogs of the aristocracy. Ancestors of many modern dog breeds are evident in medieval sources, including greyhounds, spaniels, poodles and mastiffs.
Greyhounds (a term that encompassed an array of sight hounds) were especially highly regarded and were seen as suitable gifts for princes. Greyhounds appeared in stories showcasing their prodigious intelligence and bravery.
(1/2 HD Brute)
AC:12 HD 1 HP 2
Mods: W-2/N+2/S+2
Speed: 40'
Resistance: 8
Morale: +2
Stealth +2, Perception +2
Attack, Melee: +2: Damage 1d6 piercing (bite)
Pack Tactics: The dog has advantage on an Attack roll against a creature if at least one of the dog's allies is within 5 ft. of the creature and the ally isn't Incapacitated.
S+2, D-2, C+2, I-2,W+2,Ch+2
(1/2 HD Soldier)
AC:12 HD 1 HP 3
Mods: W-2/N+2/S+2
Speed: 60'
Resistance: 8
Morale: +4
Stealth +2 Perception +4
Attack, Melee: +2; Damage: 1d4 piercing (bite)
Pack Tactics: The dog has advantage on an Attack roll against a creature if at least one of the dog's allies is within 5 ft. of the creature and the ally isn't Incapacitated.
S-2, D+2, C+2, I-2,W+2,Ch+2
(1 HD Brute)
AC:12 HD 1 HP 5
Mods: W-2/N+2/S+3
Speed: 40'
Resistance: 8
Morale: +4
Stealth +2 Perception +3
Attack, Melee: Bite 1D8 piercing (bite)
Pack Tactics: The dog has advantage on an Attack roll against a creature if at least one of the dog's allies is within 5 ft. of the creature and the ally isn't Incapacitated.
S+3, D-2, C+3, I-2,W+3,Ch+2