Rats
Rats were so much more then just pests during WWI. They would create their homes inside of dead bodies, nesting in the gut of humans. Seeing a rat was like seeing fear. It was more of a symbol then a threat to the living soldiers. Rats symbolized the lack of humanity in what they were doing, and how disgusting and horrible war really was. Instead of being buried and honored, the dead were being slowly eaten from the inside out. War cancels out any basic human rights we have, the rats show war's corrupt nature.