Not that I just found this out, but it is something I noticed; T. rex must have hunted some of the most dangerous prey ever.
-The main two ceratopsians of Hell Creek, Triceratops horridus and Torosaurus latus, each must have weighed anywhere from 5-12 tons in weight, and were armed with three horns that could prove some damage, even when its impossible for them to charge without killing themselves. They also possessed crocodilian-like armour on their flanks and might have had quills as well, which may have been used for defensive measures like porcupines. They also had bone-crunching beaks suitable for snapping more than tough plants. The other ceratopsian, Leptoceratops gracilis, could prove a threat to young tyrannosaurs, with a large bite force, omnivorous diet, and possible quills too.
-The most common ornithopod at Hell Creek, Edmontosaurus annectens, could have weighed as much as T. rex, if not more, and a stampeding herd could easily crush the predator. I can picture subadults also bucking up and kicking like horses; if that's impossible, tell me so. Thescelosaurus neglectus could only pose a small threat, possibly even to young tyrannosaurs. Might have kicked and had quills.
-The other three known marginocephalians at Hell Creek are Pachycephalosaurus wyomingensis, Sphaerotholus buchholtzae and Stygimoloch spinifer (which I consider "Dracorex" as the juvenile form of). The latter two were only eight feet long at the most, but could have easily injured a juvenile tyrannosaur, using their domed heads and spikes to ward one off. The former genus is the largest pachycephalosaur known, with the largest forms around 20-25ft long, putting it in the same length class as Torosaurus. At this size, it could still pose a threat to a hunting tyrannosaur, probably capable of breaking some bones.
-More ornithischians are found in T. rex's range. These are the ankylosaurians, and the most famous is the widely-built Ankylosaurus magniventris, and it was nearly 30-35ft long and 13,000lb in weight. With thick armor, a clubbed tail (around 100lb, if I recall correctly), and ferocious tenacity, it would have been one of the hardest prey items available. Edmontonia schlessmani is found in T. rex's range as well, and although smaller than Ankylosaurus (20-25ft), it was armored, with well, armor, that included very long, sharp, shoulder spikes that would easily gore an attacker's skin.
-We usually don't think of theropods hunting other theropods, and think of them going after herbivorous creatures, but several theropod species were possibly prey of young tyrannosaurs. Struthiomimus sedens, 12-16ft long, would have been one of these, and it was built like an ostrich. Sadly, many people portray them, as well as duckbills, completely harmless, but this couldn't be the case. Like I said, built like an ostrich. Ostriches, and their much more dangerous cousin, cassowaries, can easily kill with one kick; Cracked.com states the ostrich kicking with the force of "2.5 Mike Tysons". An ostrich is on average around 215lbs, and when you scale it up onto a 330lb dinosaur, the damage won't be pretty.
-The other theropods that are possibly tyrannosaur prey are odd creatures; the oviraptorosaurs. Chirostenotes elegans was a 6-8ft long creature and could have kicked like an ostrich as well. I don't know much about the head strength of oviraptorosaurs, but they might have been able to bite pretty hard as well and ram with their crests, as well claw with their arms. The Triebold caegnathid, unnamed at the moment, was twice the size of the largest Chirostenotes, and would have posed even more of a threat.
-One more saurischian could have possible prey for T. rex, albeit not at Hell Creek, and a T. rex would have to be suicidal to attempt to hunt it alone. Alamosaurus sanjuanensis is the largest dinosaur known from North America (besides the extremely fragmentary Amphicoelias) and was Argentinosaurus-sized, placing it at 80-90 tons. T. rex would have had to hunt in packs to bring an adult down, and even then, it was still a danger.