How do you make a dinosaur roar, when we have no idea what dinosaurs actually sounded like? The sound designers for Jurassic Park recorded living animals and combined them. In this short video, we learn a little about field recording, sampling, and the art of making an extinct dinosaur roar. Keep an eye out for the super-sweet 90s computer plus Synclavier in the audio studio.
According to this very blog in 2006: "T-Rex’s roar was a remix of sounds from a crocodile, a lion, a tiger and a baby elephant." So there you go.
If you like that, you might enjoy this TV coverage from 1993 showing a little more about the movie's sound design:
Also highly relevant: Is It True That T. Rex Could Only See Things That Were Moving? And a much longer video containing the entire "Making of Jurassic Park" documentary.