Do you think we'll encounter them in the game?
I give here a list, with few words about what i found about them. Some of them were mentioned by a Nord called http://www.uesp.net/wiki/Morrowind:Briring but since I use the French version of Morrowind, it is possible that alterations appeared in their names due to double translation. If you find such mistakes, please post the corrections, so that everybody can follow. Also, if you find other creatures (in the widest possible meaning) which were mentioned in the lore and would deserve appearance in Skyrim, please post them too.
Clockwork Dreughs
They http://www.uesp.net/wiki/Lore:36_Lessons_of_Vivec,_Sermon_36 dreugh-shaped amphibious counterparts to the Dwemeri war machines, fashioned by the Tribun Sotha Sil.
Ironwood trees, Black Ironwood trees
They are mentioned in http://www.uesp.net/wiki/Lore:Special_Flora_of_Tamriel. Black ironwood trees are very rare Skyrim-specific trees famous for their nuts. Perhaps their wood is strong enough, or has special alchemic properties which makes it suited for crafting of weapons of armor, but this is purely hypothetical from me.
Snow hawks
Perhaps it's exagerated to say they have been mentioned, but there is a city in Skyrim called Snowhawk (look at the http://www.uesp.net/wiki/Oblivion:Whispers_of_Death). Then, the hypothesis of the existence of eponymous animals lies just one step further from whence this puts us. Moreover, perhaps their introduction could allow for falconry in Skyrim, which would be a very nice stuff, very well suited to Skyrim's sphere, in my opinion.
Emperor crabs
It is not accurate that it was never seen before. In fact, it has never been seen alive before. The famous Skar of Ald'ruhn is one of them. Perhaps it would not fit in Skyrim, but well in offshore areas, such as Roscrea...
Royal mudcrabs, colossal mudcrabs
Mentioned by Briring in Morrowind. Bigger than the usual ones, and more dangerous.
Blind slaughterfishes
Mentioned by Briring. Bigger and more dangerous than the common ones. Perhaps their supposed blindness is a clue they live in deep waters, where light doesn't reach?
Electric slaughterfishes
Mentioned by Briring. Could make interesting opponents, in my opinion.
Wild Nix hounds
Mentioned by Briring. Bigger and more dangerous than the common Nix hound, it could perhaps appear near the Eastern border.
Red-haired rat
Mentioned by Briring.
Well, I am aware that if they aren't already in Skyrim by now, it isn't likely that they'll be introduced, except perhaps in DLC or expansions. It would perhaps require much work and time to make models for them, for their moves, for related ingredients and for their interactions with the existing ecosystem.
So what do you think? Should they be included?