The dog is a quadruped mammal, almost certainly descended from the same root-stock as the visually-similar wolf. Together with the cat, it became, during pre-history, the main domesticated species in Europe. It hunted and herded for man, guarded his homes, and (on occasions) was trained to fight in his wars. The cat, on the other hand, killed mice, drank milk, and looked elegant.
Spiritually the dog was seen as unfailingly faithful (hence the adjective dogged) and hard-working. For this reason, when its loyalty failed, it was noted and "dog" became a term of abuse in some circles.
King Arthur had a famous hunting dog, Cabal, who is supposedly buried on a Welsh mountain.