A meteor crashed near an American city in 1966 and a roughly humanoid alien materialised on top of it. The alien was greeskinned, man-sized but with a frightful giant head and a childish body and had the ability to turn earthly animals over ten times there normal height and under his mental control. It mentionned that this technique had already been used on other worlds to conquer them.
The destruction of the meteor that brought it made the alien vanish, leading to believe that the Alien Brain was the metero itself and the humanoid alien a mere projection/creation destined to strike fear in the humans rather than a representative of another species.
Source: Hanna Barbera's Frankenstein Jr. Episode 4 "Alien Brain from Outer Space"