GNT, yer thinking of this robot from Forbidden Planet...
Per Wikipedia, the Lost in Space Robot...is a Class M-3 Model B9, General Utility Non-Theorizing Environmental Control Robot, which had no given name. Although a machine endowed with superhuman strength and futuristic weaponry, he often displayed human characteristics such as laughter, sadness, and mockery as well as singing and playing the guitar. The Robot was performed by Bob May in a prop costume built by Bob Stewart. The voice was dubbed by Dick Tufeld, who was also the series' narrator. The Robot was designed by Robert Kino****a, whose other cybernetic claim to fame is as the designer of Forbidden Planet's Robby the Robot. Robby appears in LIS #20 "War of the Robots", and the first episode of season three; "Condemned of Space".
Found this in Science and Fantasy:
His name was "Robot". Just to nitpick, in the second season episode "Ghost Planet" he is referred to as "Robot B-9." In every other episode he is addressed merely as "Robot."
In an episode where they travel back in time to just before they launch, there is a crate in Dr. Smith's office. It contains the Robot. On the side there is an acronym, G.U.N.T.H.E.R., which stood for General Utility Non Theorizing Environmental Robot. But as others have said, they always referred to him as Robot. Some people think his name is Robbie, but that is only because he was designed by the same person: Robert Kino****a. Originally, Robbie was supposed to be the Robinson's Robot, but Irwin Allen could get him for filming the new material added to the pilot footage. Robbie was a busy robot actor back then and made many guest star appearance on various TV shows-including eventually Lost in Space, first as the Robots' adversary, in "War of the Robots" and later on in "Condemned in Space."