Correct sentence: Long ago, I was a huge T. rex.
Why this is better (easy steps):
- Capitalize the first word and the pronoun I. Every sentence starts with a capital letter, and I is always uppercase.
- Put a comma after an introductory phrase. 'Long ago,' is a short opening phrase, so a comma after it is good: Long ago,
- Write the dinosaur name as 'T. rex.' Many books use a period after the T and a space before rex: T. rex. Some people write 'T-rex' too, but 'T. rex' is the usual way.
- 'a huge' is correct. It tells how big the dinosaur was, so you don’t need to change that.
Other ways to say it:
- A long time ago, I was a huge T. rex.
- Long ago I was a huge dinosaur. (this is okay but less specific)
Nice sentence — great imagination!