The dinosaur that T-rex was afraid of was the Ankylosaurus as well as the Triceratops.
The Ankylosaurus and the Triceratops dinosaurs were the most dangerous dinosaur opponents to the T-rex dinosaur.
However, the T-rex was not likely actually afraid of any one dinosaur as in the way us humans would feel fear, but they would have also treated the heavily armored or large horned herbivore dinosaurs as being high risk and dangerous prey, which could lead to fatal injuries to them.
The Triceratops dinosaur, most specifically the full grown Triceratops, with it's 3 sharp horns could kill a T-rex dinosaur easily in a head on confrontation.
The Ankylosaurus dinosaur was a heavily armored herbivore that could easily shatter a T-rex dinosaurs leg bones with it's massive tail club and cause a slow death to the T-rex by starvation.
Another dinosaur that the T-rex would be afraid of is the Alamosaurus, which was a massive sauropod, and was a dinosaur that was likely too large for a T-rex dinosaur to handle and tackle and fight off alone, and just one stomp from an Alamosaurus to a T-rex dinosaur would be fatal.
A T-rex that had a damaged leg, meant that the T-rex dinosaur could not hunt, which made the potential reward of attacking an adult Ankylosaurus or Triceratops dinosaur not worth the risk.
And the armor plating that an Ankylosaurus had, made it nearly impossible for any T-rex dinosaur to delivery any efficient killing blow to them.