| "Ooh, ah," that's how it always starts. But then later there's running and screaming."
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This creature is never called by a given name, however Robert Muldoon nicknames her "Rexy" on multiple occasions in the first novel. This is similar to how the Big One, a Velociraptor, is also based on a nickname and not a given name in the franchise. She was also given a similar nickname by Billy Yoder, but under a different spelling ("Okay, we can't go back outside because of our old friend Rexie").
Rexy was created inside the laboratory of a village owned by InGen on Isla Sorna for their amusement park/zoo Jurassic Park on the neighboring island Isla Nublar. A Version 4.1 Tyrannosaurus, she was shipped to the park to live in the attraction known as Carnivore Country 2 sometime before 1989. Despite being female, the park's staff would refer to her as a male. Later, or possibly simultaneously, a juvenile was shipped to her enclosure to live with her, but would keep its distance from Rexy. During Rexy's time in the enclosure, she and the juvenile would sometimes get sick from drinking the water of the enclosure for an unknown reason. Rexy would also gaze at the nearby Sauropod Paddock interested in the Apatosaurus living there.
- "You know, at times like this one feels, well, perhaps extinct animals should be left extinct. Don't you have that feeling now?"
- —Ian Malcolm to Alan Grant after Rexy's attack on Land Cruiser BB4.(src)
The endorsement witnessed Rexy being fed a goat during their tour of Jurassic Park. Because many of the members were leaning out of their car windows during the spectacle, Robert Muldoon, the park's game warden, became concerned for their safety so he decided to retrieve the park's only rocket launcher.
When Dennis Nedry disabled Jurassic Park's power, Rexy was able to escape her enclosure. After the juvenile escaped, she too did the same. Using her hind limb, she tore down the depowered electric fence. Once free, she decided to attack the Land Cruiser of Tim and Lex Murphy, BB4. The Tyrannosaurus thrashed the car several times, severely damaging the vehicle and injuring its passengers before she finally lifted BB4 into the air where it became caught in a tree. She then proceeded to sniff the ground. While she did this, Dr. Ian Malcolm, who witnessed the attack with Dr. Alan Grant inside Land Cruiser BB5, decided to leave the vehicle and make a desperate run to safety, which was in the Tyrannosaurus' direction. Rexy noticed him and tossed him away violently, with the fall breaking his leg. Afterward, Rexy also noticed Dr. Grant leaving his car as well, but unlike Malcolm, Dr. Grant stood still, preventing Rexy from seeing him. But the Tyrannosaur indirectly attacked him unknowingly when she kicked the land cruiser over. Though they were injured from the attack, all the people that Rexy harmed survived the attack.
At some point, Rexy tore through the fence of the Sauropod Paddock and entered it, presumably due to her interest in the paddock's sauropods. Inside the paddock, she hunted a herd of Hadrosaurus killing one in the process, Dr. Alan Grant and the Murphy children witnessed this hunt. She then took one of the Hadrosaurus she killed and traveled to the dock near the Raft Storage Maintenance Building where she fell asleep.
Dr. Grant and the Murphys encountered the adult Tyrannosaurus while trying to retrieve a raft to reach the human populated areas of Isla Nublar. They managed to retrieve one of the rafts in storage and became waterborne without waking her. However, not too long after getting onto the water, Lex Murphy started to cough, which awoke the sleeping T. rex. Now awake, Rexy proceed to pursue the boat and attack the three. During the conflict, Dr. Grant tried to stop her by firing the air pistol at her, to no effect. Ultimately, Rexy did not kill the humans despite being close to doing so as she heard the juvenile Tyrannosaurus nearby with an Apatosaurus it had killed and instead chased the younger member of her species instead.
The Tyrannosaurus encountered Dr. Grant and the Murphys once again while on the bank of the Jungle River and tried to attack them several times, but was unsuccessful each time due to the thick foliage restraining her. Unsuccessful in her attacks, the Tyrannosaurus continued to travel down bank where she harassed a mating pair of Dilophosaurus, which allowed the humans to safely bypass the venomous dinosaurs.
The Tyrannosaur continued to explore the Jungle River Cruise and as she did, Robert Muldoon and Donald Gennaro were waiting to tranquilize her and return her to her paddock. Muldoon fired his shot at Rexy only to miss. This angered the Tyrannosaurus and she proceed to charge toward the two men's jeep. As she ran towards them Muldoon fired a second shot, which did hit its target, but because the tranquilizer's effects did not reactive fast enough Gennaro believed he had missed. After the second shot was fired, the two men quickly returned to the Visitor Center to evade the angered Tyrannosaur.
Just after the Muldoon and Gennaro left, Rexy encountered Grant and the Murphy children for a fourth time as their raft fell down the nearby waterfall that contained Maintenance Building 04. Rexy attacked them again, but only caught Lex's life jacket and the humans went inside the waterfall while she was distracted with the vest. Later, when Alan Grant was inside one of the maintenance building's interior, Rexy poked her head inside the waterfall. She used her tongue to feel around the room for any of them humans. She found Tim and began to pull him towards her mouth, but while doing so the effects of the tranquilizer kicked in, rendering her unconscious.
Jurassic Park's staff was supposed to retrieve her after they had discovered she was successfully unconscious, but the park ran out of auxiliary power just after the discovery.
Both tyrannosaurs and the raptors killed many of Jurassic Park's fauna, creating an equilibrium in Isla Nublar's dinosaur population.
If she did not drown in the pool of water made by the waterfall, Rexy was certainly killed along with the rest of the cloned fauna on Isla Nublar in the Costa Rica Napalm Bombing that followed after the InGen Incident.