Well first I think you should go about making a list of all creature of morrowind.. will it include the undead? limited to region? Will deadra be part of it?
Heres a small example...
Name: Cliffracer - Scientific Name: (If they have one)
Local: Everywhere! (thats a little over dramatized)
Prey: (what it feasts on, or you could put what feasts on it)
Statistics: (Make up something here)
Disease Possiblity's: (Like blighted, diseased, etc.)
Warnings: (Like are they hostile and such)
Misc. Facts: (..doh)
--
I hope that helps a bit

If you need more help I'd be more than willing to do so