Ratels and their closer relatives are the only animals crazy enough to engage in a 1 versus 1 against a T-rex...not even lions fight alone, and tigers only kill things weaker than them. And any other animal would just turn back and run away from a T-rex...there's no animal on earth that wouldn't crap themselves in fear when confronted by such a monster.
But what about snakes? Snakes are even afraid of humans, they'd probably die of a heart attack when a tyrannosaur is charging them, even if the T-rex is allergic to bees, the bee would die after stinging the dinosaur. And other poisonous insects don't have the guts or the strength to pierce the dinosaur skin in order to inject their poison, but even if they were able to pierce a dinosaur's skin,they would not have enough venom.
Only poisonous frogs have enough toxins to kill a tyrannosaur, but the frog would need to be eaten alive in order to kill the dinosaur.
Ratel and it's closer relatives are the only animals frequently engaging combat with other predators bigger, faster, smarter and stronger than them.
There are also accounts of wolverines ( the same family of Ratels) killing polar bears by biting their throat.
The way I see it, there's literary no other animal that would actually fight against a T-rex. And the only way your wolverin or ratel has to win is to get between the tyrannosaur legs without getting stomped, climb up and bite it's eyes out off the tyrannosaur skull. T-rex has no arms to defend itself, and once the ratel started climbing, the odds are in favor of the ratel.
The T-rex could try rolling on its back to fend off the ratel, but those animals are agile and the ratel could easily jump off before being stomped, and while the tyrannosaur is still on the ground, the ratel has an advantage reaching the enemy skull.
Could a ratel just bite a T-rex throat and suffocate it by leaching onto it for a few minutes? Probably, but the tyrannosaur would have some time to think about ways to kill the small ratel in those minutes.
If the ratel just bites off the T-rex eyes, everything becomes safer and the ratel could even reach deeper into the skull and eat their brains out.
Also a T-rex's eye is as big as a Ratel's head, so the small guy would have no problem jamming it's teeth through.