The dog's oldest mythological employment is at the border of death. Anubis, the jackal headed guide who weighs hearts; the dogs of Hecate at the crossroads; Cerberus at the gate of Hades; the dogs of Yama in the Vedas: across traditions the canine keeps or crosses the threshold between worlds, which folklorists connect to the dog's real work at the boundary of camp and wild. Folk belief kept the association domestic, in the widespread omen that a dog howling by night foretells a death, and the graveyard black dog of British lore. A dream dog stands in that lineage as well as at the foot of the bed.
Against the psychopomp stands the companion. The dog is the first domesticated animal, and its dream conventions are correspondingly warm: loyalty, protection, friendship without conditions, the folk dictionaries reading a friendly dream dog toward faithful friends and a snarling one toward friendship soured. The Jungian tradition reads the dog as instinct on good terms with the dreamer, the animal nature that serves and warns rather than threatens, with the hostile or rabid dream dog as that relation gone wrong.
Interpreters weigh the dreamer's own history with dogs first, love or bite, before any symbolism, and then the dream dog's behaviour, guiding, guarding, following or barring the way. That last office, the older traditions would note, was always the dog's proper work.