"What human being more clearly or so vociferously gives warning of the presence of a wild beast or of a thief as does the dog by his barking? What servant is more attached to his master than a dog? What companion more faithful? What guardian more incorruptible? What more wakeful night-watchman can be found? Lastly, what more steadfast avenger or defender?"
Columella, De Re Rustica (VII.11.1)
This melancholic portrait of a dog next to an overturned pitcher probably dates from the first century BC and was found during excavation of the new Alexandria Library (Egypt). It now is in the Greco-Roman Museum there.
Reference: The illustration is from Antike Bildmosaiken (2003) by Bernard Andreae.