Big Brain only appears in Fairly Odd Parents episode named "Future Lost". It is a giant brain that could be considered a Skynet's parody. He refers to humans as "meat puppets" although he, being a brain, is also organic. He is tucked in a huge machine to communicate, absorb brain nutrients and control robotic minds.
Biography
Timmy Turner reads his favorite science fiction book Steve Sparks and, believing it is a simpler world, he wishes his was like it. In that new reality people live both with robots and aliens and, for that reason, Mark Chang can walk among them without attracting attention. Further Crocker is a concierge with a big muscularity because of the hardness of him new work and is bullied for thinking that "robots are evil".
Later Timmy suggests to Cosmo and Wanda to suit becoming robots but turns out to be a mistake, because robots rebel and Cosmo and Wanda join them thus preventing Timmy could want the world returned to normality. Timmy, after have forcibly waxed many robots, read his book to find solution. In it Steve Sparks flies to Antarctica to fight against Big Brain, rebellion robots causative.
Timmy flies with Crocker and Mark Chang in the nave of the latter to Antarctica and plan the strategy but Timmy and Crocker are captured and brought to Big Brain. After discovering Big Brain's capacity to absorb nutrients Timmy takes one of his futuristic nutrients and with athletic, though sick, help of Crocker breaks the jar which is tucked Big Brain and throws a freezing nutrient inside plunging the evil brain in a state of brain freeze, thus freeing all robots in the world from its electronic control. Cosmo and Wanda are free too and Timmy finally wishes world returned to normality.