An modern animal hunting in packs might bring T-rex down. Imagine the shallow-water fight again, but this time, instead of one, or even several orca’s, imagine a large group of sharks. And sharks don’t even work well together, they just combine forces.
Mammoths were pretty large as well, but they were brought down by humans hunting in packs. Just imagine the hunting lore 'When Grandpa brought down a T-rex!"
To expand on QtM’idea, how about a T-rex with a several large flesh wounds from, infected with modern maggots?
Thanks yBeayf for the links. I knew about narwhals as a source for unicorn legends, but your Elasmotherium is a much more likely candidate.