Start working it out like a ref sheet.
You need the common, rare, and exotic variants of your creature, so work that out early on.
What is the species name?
What does the species do?
Where do they live?
Do they live in packs, civilizations, alone?
What time period would they be found in?
Do they have any sort of mythology or lore?
I’d consider all of these questions and more. Hope this helps!!