> Yet, go back far enough and you'll find T-Rexes in the mix

Afaik, T-Rex was never a direct ancestor of modern birds, including chicken. T-Rex and birds are theropod dinosaurs, but it was a very large and diverse group of animals.

But as soon as you've gone up from a chicken to the ancestor of a T-Rex, you do indeed find that T-Rexes are in the mix. They look different from what you'd normally think of as a T-Rex, in the same way that they also look different from what you'd normally think of as a chicken.