Theo made his debut in T-Rex Returns. He was first seen feeding on foliage when an injured T. Rex proceeds to attack him. Nigel then thought he could rescue two species for the price of one, by rescuing Theo and the Tyrannosaurus rex. Unfortunately, the Tyrannosaurus didn't take the bait. The Triceratops, on the other hand, went through the portal, and was declared the park's first ever prehistoric creature. Bob eagerly transports Theo to his new enclosure, Triceratops Creek. Later on in the episode, Bob watches the monitors, and notices Theo charging into a tree in his enclosure. Bob & Suzanne both head down to see what is troubling the Triceratops. Suzanne notices that Theo's frill has changed color, and believes that he is experiencing a surge of testosterone. Bob then comes up with a private plan to calm down Theo. After Nigel returns from the T-rex rescue, Suzanne tells him to check out Theo. As the two observe Theo, he charges right at them, only stopped by the fence. Nigel believes that Theo is experiencing a testosterone surge, by comparing Theo to a time when Nigel hand-reared a female baby fox that grew little aggressive as it matured. Suddenly, Bob shows up on a tire-padded tractor made to look like a Triceratops. He tells Nigel to hop on and tells Suzanne to open the gate. Bob & Nigel then enter the enclosure and joust with Theo, until the adolescent dinosaur has calmed down. Theo was then able to settle in.
In Supercroc, Theo wasn't focused on much, but was doing well in his enclosure. Unlike many of the animals, Theo doesn't participate in the Mass Break-Out, but he did witness Nigel get chased by Matilda.
2 Triceratops were in the park in the opening titles even though Nigel never brought more than one back to the park in the original series, implying that after the events of "Supercroc", he rescued one more Triceratops for Theo.