They’re called paradummies and were first used during World War II. Their purpose was to make an air invasion seem much more daunting than it actually was, and they were extremely successful. While both sides used the dummies, the British were the ones who perfected the technique.
The most famous time paradummies were used was on June 6, 1944 called Operation Titanic. In the early hours of the morning 500 dummies were dropped in four different locations. Attached to the dummies were rifle fire simulators and two teams of Special Air Service soldiers who carried loud battle noise recordings.
Their intent was to push German troops away from the Allies actual drop zones, which they accomplished. Unfortunately, only two of the soldiers that were not dummies escaped when the Germans rushed at who they assumed were enemy soldiers.