During the Cold War, American firms got funding for just about any crazy idea they had. In the 1960s, the Army was looking for a way to improve mobility, knowing that wheeled and tracked vehicles were unable to work in some climates. The Army called up General Electric, and told them to get cracking on a walking truck. Yes, a walking truck.
It was an ambitious design, using four legs to simulate the way animals walked. Essentially, the Walking Truck was a cybernetic horse. At eleven feet tall and driven like a tank, the Walking Truck was an imposing sight, and General Electric thought they had revolutionized military transportation. Testing, however, soon unveiled key issues. First, it could only reach a top speed of five miles per hour, which was way too slow for a combat vehicle. Secondly, it had huge problems with stability, necessitating awkward stabilizers stuck off to the sides. Essentially, the Walking Truck had the same issues as the AT-AT walkers in Empire Strikes Back. The Soviets just needed harpoons and tow cables to bring them down.
Obviously, the Walking Truck never caught on, but it did capture the imagination of the special effects designers for Star Wars. Inspired by images of the Walking Truck, the production crew of Empire Strikes Back came up with the AT-AT walker. This is one of the few times in Star Wars that the tech was based on something that really existed.