I want a
flaming frost troll.
Hows that for unique?
On a more serious note, I think the one of a kind creatures should be very difficult to find, i.e. the giant slaughterfish in OB, it was impossible to get to unless you had a water breathing spell or were playing an Argonian. There should be rewards like that for people who explore dungeouns to their full extent, sort of like the seceret areas in DOOM, though not as hard to find.
As for rare species, their should be about 8 or 10 of them in game, and they should also be difficult to find, but not nearly as difficult as the one of a kind monsters.
Also, for any unique or rare monsters, there should be some kind of souvenir you can pick from their bodys, like a horn, head, or hide.
I want a wolly mammoth rug!