Boston Dynamics’ four-legged robot developed for the U.S. military in 2005 was called BigDog.
BigDog was designed to carry equipment over rough terrain where wheeled vehicles could struggle. It used hydraulic actuation, onboard sensors, and balance-control software to walk, climb, and recover from disturbances. Videos of the machine staying upright after being pushed made it one of the most recognizable research robots of the 2000s.
BigDog was funded mainly through the Defense Advanced Research Projects Agency, or DARPA. It was a research prototype rather than a mass-produced military product. Its loud engine and complex hydraulic system also limited its practicality in some settings.
Boston Dynamics later developed smaller and quieter quadrupeds, including Spot. Spot is a commercial robot, while BigDog is best remembered as an experimental platform that advanced dynamic legged locomotion.