This article is non-canon.
The subject matter of this article does not take place in the "real" Half-Life universe.
The Zentraedi Tactical Battle Pod, or Polyrobo (sometimes called Walking Mech) is a robot model used in a GoldSrc skeletal animation demonstration and seen in the Half-Life Alpha, designed to stress and test the engine.
The demonstration was shown to Next Generation in August 1997. It was intended to show the comparative processor requirements between standard vertex animation and skeletal animation; with skeletal animation being massively less processor-heavy, as presented in the demonstration.
The robot, having 6000 polygons, chrome textures, and only 20 bones, was intended to crash the engine, however, it did not. Shooting it will cause it to "dance" for a while then stop. The dance the robot is doing is believed to be based on the old "Ooga Chaka" dancing baby.
Its model is based on the Zentraedi Tactical Battle Pod from the anime Super Dimension Fortress Macross.