You quote only your example, not what you are directly saying to us."T-Rex would have been the most fierce hunter the world has ever known." (I want to ... )
T-Rex would have been
"T-Rex could have been the most fiercest hunter that has ever existed."
Does "has ever existed" work with "would have been?" I think you are saying that if I have to use "has ever existed", then I will have to use "present tense" without "could/would have done". Am I correct?You need a space after the "T.", before the "rex".
The two halves don't really work together. If you use "has ever existed" you are including the present day. "Could have been" doesn't work in that case.
The T. rex could be the fiercest hunter that has ever existed.