Rexy is the nickname given to the Tyrannosaurus rex that appears in Jurassic World.
The adult Tyrannosaurus is the same individual commonly identified as the original film’s principal tyrannosaur. In Jurassic World, she escapes during the Indominus rex crisis and battles the engineered hybrid near the park’s lagoon.
The name “Rexy” is widely used by fans and franchise reference material to distinguish this Tyrannosaurus from other individuals. Her return connects the 2015 film to the 1993 original and makes her one of the franchise’s most recognizable dinosaurs.
Buck and Doe are Tyrannosaurs from The Lost World: Jurassic Park, while Big Eatie is a different Tyrannosaurus introduced in Jurassic World Dominion. Those names refer to separate animals, not Rexy.