The Cybertronian walker is an unknown walker/beast used by either Autobots or Decepticons. It is only seen during the flashback at the beginning of The Dark of the Moon.
The walker itself has a head, four legs, a tail, and two huge launchers on it's back. It has a machine gun mounted under its head, and the two legs in front of it have claws. The walker itself may be alive, but it can also be controlled by either Autobots or Decepticons. It is unknown which side it is on, since Optimus doesn't say which ones are the Autobots or Decepticons during the flashback, because they all look the same.
The walker is massive, and it towers over other Cybertronians. It is at least 3 times as tall as Shockwave. It may be possible that there is more walkers, but only two are shown. They only are seen on Cybertron.
Two walkers are seen during a flyby of the Ark during the Cybertron flashback scene. One can be seen below on top of a bridge, fighting against other Cybertronians. Unfortunately, a missile fired by one of the others hits a walker and it falls and plummets below. It is not seen again.