MIT's 3D One-Leg Hopper robot was built in about 1983; it was a tethered monopod robot used for experiments on balance and dynamics in legged motion.
Compressed air provided motive power, hydraulics provided hip movement and it could hop at up to 4.8 mph. It was even shove-resistant, as shown in this video about our old friend BigDog.
Here, however, in glorious LD (low definition), is MIT's 3D One-Leg Hopper robot:
(MIT's 3D One-Leg Hopper robot video)
I seem to recall that Bruce Sterling used this idea in his 1994 novel Heavy Weather:
Then she saw it too. A bouncing machine...
It was crossing the hills with vast, unerring, twenty-meter leaps. A squat metal sphere, painted in ragged patches of dun and olive drab. It had a single thick, pistoning metal leg.
(Read more about Sterling's mule robot)
I'm pretty sure that there was a patent for a one-legged hopping robot that dates back to the 1940's, so it's not a new idea.
Update: Thankfully, readers have a better knowledge of my database than I do. In his 1954 novel Lucky Star and the Oceans of Venus, Isaac Asimov writes about the hoppers:
Hoppers are probably the most grotesque forms of transportation ever invented. They consist of a curved body, just large enough to hold a man at the controls. There was a four-bladed rotor above and a single metal leg, rubber-tipped, below. It looked like some giant wading bird gone to sleep with one leg folded under its body. Lucky touched the leap knob and the hopper's leg retracted. Its body sank till it was scarcely seven feet from the ground while the leg moved up into the hollow tube that pierced the hopper just behind the control panel. The leg was released at the moment of maximum retraction with a loud click, and the hopper sprang thirty feet into the air...