Animals have been used for military functions for millennia. Horses, elephants, camels, and other animals have been used for both transportation and mounted attack.

Pigeons were used for communication and photographic espionage. Many other animals have been reportedly used in various specialized military functions, including rats and pigs. Dogs have long been employed in a wide variety of military purposes, more recently focusing on guarding and bomb detection, and along with dolphins and sea lions are in active use today.
 
The horse was the most widely used animal throughout the recorded history of warfare. Early mounts could pull a chariot or carry lightly armoured skirmishing forces. With the appearance of heavier mounts and the invention of the stirrup, the horse-mounted cavalry became the most prestigious combat arm in Europe for several centuries. A knight's warhorse was trained to bite and kick. The combination of the horse-mounted warrior armed with a bow made the steppe people's armies the most powerful military force in Asian history.

​With the appearance of modern ranged weapons and motor vehicles, horse use for military purposes fell into decline. However, horses and mules are still used extensively by various armies today for transport in difficult terrain.

Mules were used by the U.S. Army, the British Army, and the British Indian Army during World War II to carry supplies and equipment over difficult terrain. Pack animals that are innately patient, cautious, and hardy, mules could carry heavy loads of supplies where Jeeps and even pack horses could not travel. Mules were used in North Africa, Burma, and in Italy. They are also used for transporting supplies in mountainous regions.
 
Both Sweden and, later, the Soviet Union, attempted to utilize moose as deep-snow cavalry. Moose were discovered to be unsuitable for warfare, as they easily contracted livestock diseases, were difficult to feed, and fled the battleground. The Soviets later trained moose not to be gun-shy, but were unable to make use of their cavalry because of the Soviet-Finnish War and World War II.
 
Dogs were used by the ancient Greeks for war purposes, and they were undoubtedly used much earlier in history. The Roman Empire, starting with Marcus Aurelius, also used dogs in combat. The Romans trained the Molossian dog specifically for battle, often cladding them in protective spiked metal collars and mail armour, and arranging them into attack formations. During their conquest of Latin America, Spanish conquistadors used Alaunts to kill warriors in the Caribbean, Mexico and Peru.

​Mastiffs, as well as Great Danes, were used in England during the Middle Ages, where their large size was used to scare horses to throw off their riders or to pounce on knights on horseback, disabling them until their master delivered the final blow.

Animals as living weapons
 
Historical accounts of incendiary pigs were recorded by the military writer Polyaenus and by Aelian. Both writers reported that Antigonus II Gonatas' siege of Megara in 266 BCE was broken when the Megarians doused some pigs with combustible pitch, crude oil or resin, set them alight, and drove them towards the enemy's massed war elephants. The elephants bolted in terror from the flaming, squealing pigs, killing great numbers of their own soldiers by trampling them to death.
 
According to Pr. Shi Bo, monkeys were used in the beginning of the Southern Song dynasty, in a battle between rebels of the Yanzhou (Yasuo) province and the Chinese Imperial Army, led by Zhao Yu. The monkeys were used as live incendiary devices.

​The animals were clothed with straw, dipped in oil and set on fire. They were set loose into the enemy's camp, thereby setting the tents on fire, and driving the whole camp into chaos.
 
In 1267, the sheriff of Essex was accused of plotting to release flying cockerels carrying bombs over London.
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While elephants are not considered domesticable, they can be trained to serve as mounts, or for moving heavy loads. Sanskrit hymns record their use for military purposes as early as 1100 BCE. A group of elephants was employed by Hannibal during the Second Punic War. They were employed as recently as World War II by both the Japanese and Allies. Elephants could perform the work of machines in locations where vehicles could not penetrate, so they found use in the Burma Campaign.
 
Camels have typically seen use as mounts in arid regions (camel cavalry). They are better able to traverse sandy deserts than horses, and require far less water. Camels were employed in both world wars. Camels are used by the Indian Army and Border Security Force for patrolling in the desert regions of Rajasthan.
 
In World War II, many military units of the Soviet Red Army, sometime after the Battle of Stalingrad, took to using camels in the southern theatre of the war in order to transport ammunition, fuel for tanks and aircraft, food, water for kitchens, fuel, and even wounded Red Army soldiers. The use of these animals as means of transport was made necessary by the Kalmyk steppes' open terrain, its primitive roads and lack of water, as well as a shortage of adequate auxiliary vehicles in the Soviet armed forces.

Ramses II had a pet lion which fought with him during the Battle of Kadesh.
 
Pliny the Elder wrote about the use of pigs against elephants. As he relates it, elephants became scared by the squeal of a pig and would panic, bringing disaster to any soldiers who stood in their path of flight.
 
War elephants were widely used in most parts of South Asia and North Africa, and were also employed by the Diadochi kingdoms, the Kingdom of Kush and the Roman Empire. They were usually equipped with armour and tusk swords and carried a howdah with soldiers and were controlled by a mahout. The Khmer Empire used ballista elephants, war elephants equipped with ballista-like weapons.
 
Cattle, in some instances, saw use in battle through manmade stampedes of panicking herds that were driven towards the enemy. This was often a dangerous tactic to those that released them, especially following the invention of gunpowder. In both the Battle of Tondibi and Henry Morgan's siege of Panama, herds of cattle were released onto the battlefield, but stampeded back into their own army's lines due to gunfire from the opposing army.
 
During the Warring States period of Chinese history, Qi general Tian Dan is said to have had 1,000 oxen dressed and painted like dragons, then their tails lit on fire and sent against the opposing Yan army.
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Even more bizarre:

• Anti-tank dogs – a Soviet, World War II weapon that had mixed success. Canines with explosives strapped to their backs were used as anti-tank weapons.
• Project Pigeon – a proposed U.S. World War II weapon that used pigeons to guide bombs.
• Bat bomb, a U.S. project that used Mexican free-tailed bats to carry small incendiary bombs.
• Animal-borne bombs have been used by modern terrorists and insurgents in the Middle East, who have affixed explosives to animals, sometimes left wandering alone and other times ridden by suicide bombers, in modern insurgent attacks in the Middle East.
• Exploding rat– dead rats were prepared for use by the British Special Operations Executive in World War II against Germany. Rat carcasses were filled with plastic explosives, to be left in locations such as factories where, it was hoped, the stoker tending a boiler would likely dispose of the unpleasant discovery by shovelling it into the furnace, causing it to explode. The rats contained only a small amount of explosive; however, a puncture of a high-pressure boiler could trigger a devastating boiler explosion.
• Animals' carcasses have been used to camouflage roadside improvised explosive devices during the Iraqi insurgency.

Deception and psychological warfare
 
In the Battle of Pelusium (525 BCE) between the Achaemenid Empire and Ancient Egypt, Polyaenus claimed that the Achaemenid forces held cats in front of them as a psychological tactic against the Egyptians, whose archers did not shoot for fear of harming what they considered to be sacred animals.
 
In the Battle of Ager Falernus (217 BCE), Hannibal Barca had torches attached to the horns of bulls and set alight during nightfall before allowing them to run amok. The Romans, believing the torchlight was from fleeing Carthaginian soldiers, pursued the cattle and were caught in an ambush.

In communications
 
Homing pigeons have seen use since medieval times for carrying messages. They were still employed for a similar purpose during World War I and World War II. In World War II, experiments were also performed in the use of the pigeon for guiding missiles, known as Project Pigeon.

​The pigeon was placed inside so that they could see out through a window. They were trained to peck at controls to the left or right, depending on the location of a target shape.

For locating hazards
 
Dogs have been used for detecting mines; they were trained to spot trip wires, as well as mines and other booby traps. They were also employed for sentry duty, and to spot snipers or hidden enemy forces.
 
On land, giant pouched rats such as the Gambian giant pouched rat, have been tested with considerable success as specialised mine detecting animals, as their keen sense of smell helps in the identification of explosives and their small size prevents them from triggering  and mines.
 
Chickens were used during the Gulf Wars to detect poisonous gases in an operation called Kuwaiti Field Chicken (KFC); the designation of the US Marines or chickens used in this role was Poultry Chemical Confirmation Devices. The plan was put on hold after 41 of 43 chicken used for such purposes died within a week of arrival in Kuwait.
 
Beginning during the Cold War, research has been done into the uses of many species of marine mammals for military purposes. The U.S. Navy Marine Mammal Program uses military dolphins and sea lions for underwater sentry duty, mine clearance, and object recovery. A secret 1960s CIA project known as Project OXYGAS envisioned dolphin saboteurs trained to attach explosive devices to enemy ships.