Nice sentence! Here are simple steps to check and improve it.
Original: I was a huge T. rex once! Long ago.
What is okay:
- "was" is past tense, so it is correct for something that happened long ago.
- "T. rex" is written correctly.
One small issue: "Long ago." is a sentence fragment (it does not have a verb). Writers sometimes use fragments for style, but for normal grammar it is better to connect it to the first sentence.
Corrected options you can use:
- Combine into one sentence: I was a huge T. rex once, long ago.
- Start with "Once": Once, I was a huge T. rex.
- Keep two sentences but add words: I was a huge T. rex once. It was a long time ago.
If you are writing a story and want a dramatic feeling, your original is okay. If you want standard grammar, pick one of the corrected options above.