The Tyrannosaurus is one of the antagonists of The Little Mermaid episode, Land of the Dinosaurs, and arguably the true main antagonist.
After discovering some frozen dinosaurs in the North Pole, Ariel decides to use her father's trident to release them. Once free, the Tyrannosaurus immediately fights against a Stegosaurus and accidently hits the trident, who falls into the water and land in a pit. He then attacks Ariel and Flounder who flee under the sea, but he manages to follow them, apparently being able to breathe underwater. Finally, they take refuge in the pit while the Tyrannosaurus leaves towards Atlantica, followed by the other dinosaurs.
Shortly after, he encounters King Triton and Sebastian and crushes their chariot, almost killing the crab in the process. Then, the dinosaurs arrive in the city and go on a rampage. The Tyrannosaurus causes havoc and devours everything in his path. However, a family of whales suddenly comes and scares away most of the dinosaurs. The Tyrannosaurus attempts to fight one, but he is easily defeated and flees away.
Later, all the dinosaurs return to their island and King Triton uses his trident to melt the ice around, making the place habitable for them.