An arrogant and spiteful boy who takes pride in his intellect and scientific achievement, Terrance believes his fellow students are insolent "plebeians" who are below him, more concerned about science than human safety. He was the boys' rival at the South Park Elementary Science Fair in the third grade, working with Bill Allen and Fosse McDonald to clone a human, an experiment that lead to his father spawning the Evil Clone Stan, whom Terrance freed arrogantly and set on a rampage. Since his failed attempt against the boys, Terrance has receded from prominence, but has recently joined their game of Cowboys and Indians with his own contraption.
- "Hey dumbasses. You morons give up on your stupid science fair project yet?" (An Elephant Makes Love to a Pig)
- "Boys, Mr. Garrison, fellow students, for our science fair project Bill, Fosse, and I have spawned a creature genetically far superior to man. I present to you, the five-assed monkey." (An Elephant Makes Love to a Pig)
- "My vengeance set me free." (South Park: Phone Destroyer)
Terrance's father is South Park's resident mad scientist, Dr. Mephesto, and Mephesto's adopted son Kevin is therefore Terrance's brother. He looks up to his father fondly and believes him to be the best. His mother has not been seen to date.
Terrance was the boys' original rival, before Craig and Those Guys came to the forefront, and while we're not sure what he thinks of them now, it's probably not too nice.
Dr. Mephesto has always been seen at the South Park Genetic Engineering Ranch, usually with his assistant Kevin, but Terrance has only been seen there once. The place has therefore been implied strongly to be the former's residence (though no 'living space' outside the laboratory has been seen) although whether or not Terrance works or resides there is speculative.
- An Elephant Makes Love to a Pig (s01e05; debut)
- Raisins (s07e14; cameo)
- Breast Cancer Show Ever (s12e09; cameo)
- Funnybot (s15e02)
An icon for him (labeled Terrence) is included in the game's files, but he was cut from the final game. His possible role is unknown.
He is a rare character in the mobile game, where he uses a small device with spinning blades and large engines to perform as "an aerial bomber who deals heavy damage". He was one of the first characters revealed. He also appears during the Character Creation process in his father's laboratory, alongside his brother Kevin.
- Matt Stone recorded new dialogue for the character in this game, but his voice is pitched down, similarly to Fosse McDonald.
- While Terrance makes cameo appearances in the background of various scenes to this day, he has only ever spoken in "An Elephant Makes Love to a Pig".