Humans have used animals as weapons for thousands of years. 2,000 years ago, Hannibal led the Carthaginian army riding war elephants to fight Rome. In reply, the Romans set pigs on fire and let them run free through enemy ranks to frighten the elephants.
In World War II, British Special Ops thought of stuffing dead rats with explosives and spreading them throughout Germany. They hoped that the Germans would gather the rats and dispose of them in industrial furnaces, causing explosions powerful enough to trigger catastrophic boiler failures. However, the Brits dropped the plan after their first shipment of explosive rats was intercepted by Nazi forces in 1941.