Monkeys are a kind of primate with many distinct species found throughout the world, usually located in tropical habitats. Allied, Soviet, and Yuri forces often encountered monkeys in such environments during the Third World War, often with violent results.
When the Third World War truly became a global conflict with the Allied push of Soviet forces out of the American mainland, various skirmishes and conflicts began to emerge all over the planet. Tropical regions, such as isolated islands and rainforests, were hotbeds for guerilla warfare, with battles often leaving the once pristine wilderness scarred and battered beyond recognition.
However, the steaming jungles were not lacking in the ability to fight back. Many of the tropic regions occupied by war time soldiers were usually inhabited by various species of primates, among them many types of monkeys. Easily agitated by the loud noises and grinding dins produced by the destructive armies, many monkeys were spurned into violence and attacked whatever they saw walking on two legs. Soldiers on both sides often returned to their respective bases with numerous bite marks and claw marks left by the primitive berserkers, sometimes running away from barrages of thrown dirt, nuts, and feces flung from the treetops. These events became common enough that rain forest battalions were usually briefed on how to spot, seek out, and eliminate monkeys while trudging through the dense foliage.
Monkey attacks claimed few casualties, but still had the capacity to be lethal if soldiers were caught off guard. Many monkey species were made endangered by their constant elimination by human forces and took years to recover after the war had ended.
Being naturally violent towards humans, monkeys were considered liabilities to be eliminated on sight by all factions of the Third World War and were never seen as anything else. Yuri psychics could mind control them, but this was something to be done for amusement and served no greater purpose.