Mr. Rex is, like his daughter, a Tyrannosaurus rex. However, while Tina is shown to be taller than almost all students at Elmore Junior High, Mr. Rex is shown to be taller than most people in the city of Elmore, towering above Gumball, Nicole, and even his own daughter.
Like Tina, he has thick, brownish-green scales all over his body and huge, black claws on his feet and presumably his hands as well. He has an enormous mouth filled with rows of razor-sharp teeth, and piercing, red and yellow eyes. Mr. Rex's eyes also seem to be closer together and more forward-facing than Tina's. In "The Routine," it is revealed that he has a scar over his right eye. His right eye is also blinded. It is possible he blinded his eye in his fight with Nicole, as he was shown to have both eyes in "The Fight."
Mr. Rex is extremely fierce and protective of his daughter and his home. Unlike his daughter, he is shown to be very animalistic and brutal, preferring to settle disputes by fighting instead of talking.
- "The Quest": He is seen at the end of the episode, coming into frame and roaring loudly just as Tina mentions that he eats trespassers.
- "The Fight": He is seen at the junkyard when Nicole decides to come have a "talk" with him, which resulted in a fight, in which he received a massive, humbling beatdown, enough to break his house.
- "The Flakers" (mentioned)
- "The Routine": The first episode in which his entire body is in view. He is guarding a mountain of stolen groceries in the junkyard, where Richard is trying to get mayonnaise. They then get into a fight.
- "The Blame" (mentioned)
- "The Fury": He can be seen in the audience watching the martial arts tournament.
- "The Rex": This would have been the first episode in which his entire body was in the view of the camera.
- If one listens very carefully to his roars, a small portion is taken directly from the movie monster Godzilla and the T. rex from Jurassic Park.
- Mr. Rex was going to have his first major role in "The Rex," which was scrapped due to it being too expensive and not worth the money. The closest he would have to a major role would be "The Routine."