Throughout the novel, the Juvenile is said to spend most of its time staying away from Rexy and it even attempts to try and catch fish that swim around the moat surrounding the paddock.The juvenile supposedly walks past the two tour vehicles when a great thump is felt and a huge shadow blocks the view. This meant that the Juvenile was the one who broke down the fence in the first place. Ed Regis is shown to have been devoured by the Juvenile after he cowardly escapes from Rexy, the Juvenile leaving nothing more than a leg for Robert Muldoon and Donald Gennaro to discover.
Near the end of the novel, it is seen by Alan Grant in the helicopter as they leave Nublar. The Juvenile appears to have killed a Hadrosaurus and looks up at the helicopter, roaring against it. The animal was killed during the napalm bombing.
It was presumed that the juvenile T. rex didn't exist in the film canon since it never appeared. However, the JurassicWorld.com park map mentions the original Tyrannosaur Paddock. The description says that the paddock was built to safely enclose and display an adult and juvenile T. rex. It can be assumed they mean this individual.