This is a casualty dog - they were trained to find wounded or dying soldiers on the battlefield. They carried medical equipment so an injured soldier could treat himself and they would also stay beside a dying soldier to keep him company. Dogs were some of the hardest and most trusted workers in World War One.
Solders living in trenches encountered millions of pests during the war including rats. They fed on rotting food because there was no proper way of getting rid of rubbish in the trenches. A little terrier dog shows off its catch after a 15-minute rat hunt in French trenches in September 1916.