Role in the Film
He first appeared walking behind a hill in front of the humans. They run off and the Tyrannosaurus fights a Stegosaurus and kills it after a brief fight.
It followed the humans up the mountain and killed the Allosaurus the humans had intended on killing. He leaves a footprint and when the humans find it, they go back to their cave, realizing the T-Rex has followed them. The commander, Lee, reassured the group that the T-Rex could not follow them.
The T-Rex proved Lee wrong when he attacked the humans at night and killing Derna when she tries to get the laser to fight off the giant. Her death prompts one of the humans, Jim, to try to kill the dinosaur.
They first try to poison him by smearing poisonous berries on a dead Polacanthus for the T-Rex to eat. The plan backfires when the T-Rex shows up behind the group and kills Mike when he tries to fight it off.
Jim then comes up with the idea to impale the T-Rex on several wooden stakes. The T-Rex comes out to hunt before the humans can prepare the stakes but Lee leads him to an alien that resembles a smaller version of the infamous Rhedosaurus, which the T-Rex kills and brings to his cave. When the stakes are prepared, the group taunt the T-Rex outside it's cave and provoke it into chasing them. The T-Rex was so eager to catch the humans that he did not notice the stakes in front of him and ran right into them, impaling him and finally killing him.
The alien t.Rex was vicious, misanthropic, greedy, and very determined.Unlike earth t.Rex, it was more than willing to kill dinosaurs and leave the body, suggesting it killed for sport. It had traces of cunning, knowing to avoid poisened carrion, and surprise human prey, yet was stupid enough to impale itself.
The fact it is an alien that just happens to look just like a dinosaur from earth is a wink to the fact many aliens in popular culture look extreemily human.
The t.rex preyed upon a downsized rhedosaurus relatvie ease, a fictonal species that also appeared in the movie, the beast from 20,000 fathoms, but in that movie, it was ironically enormous and big enough to have eaten a t.rex.
It lacked feathers, walked with a vertical stance, had zombie hands, and had a wiggly tail it dragged, unlike earth t.rexes. This reflects older ideas about how t.Rex looked, moved, and acted.