(PS all of the beasts you mentioned were also in Daggerfall. Except Grendel.)
No, that honour would go to the kwama and its http://www.elderscrolls.com/images/creatures/mw_02CRB.jpg. Silt striders make a pretty impressive sight towering above the buildings when you step off at Seyda Neen, and it's sometimes fun to watch the betty and bull netches playing over the waters. Guar are just so [censored] adorable you can't say a bad thing about them. The other bipedal dinosaurs were a bit meh, but still all right. Nix hounds were a visual delight, though. I mean, you don't know whether it's going to lick your face or spit acid into it! That's cute!
Yeah, the kwama are definitly the most awesome creature in TES overall. :foodndrink:
However, can't agree with you about the other. Except silt striders. And the netch.
They were pushed into that by the fans who complained about the lack of fantasy wierdness in Oblivion itself. And to be honest, the things Shivering Isles weren't all that amazing. Sure, it was a great expansion and i loved it, but look at the huge murshrooms, we had those in Morrowind, the entire landmass of Dementia looks very simulair to the Bitter Coast region of Vvardenfell, Mania is a hippy Ascadian Isles region, Gnarls are a spin-off on Ents, Elytra are just huge bugs, Grummites, Scalons and Balliwogs are mutated frogs and most of the Daedra were already featured in earlier games.
Well, the same could be said about Morrowinds creatures. Most of them are just giant bug. The others are big-headed lizards and dinosaurs. The Cliffracers are "Pterodactyles". The Netch is just mutated jelly fish. Although flying, I guess
And by the way, while I don't know about Arena, the giant mushrooms were in Daggerfall as well (From what I've heard, there were also special mushrooms in Redguard that you could bounce on). They weren't special to Morrowind.
The point is, there is hardly anything left than can be rightfully called "original". While I agree that the devs got kind of lazy with Oblivions beastiary, all I would ask for is that they would have, for example, taken a pig and a bird, merged them and called it a Sweenay-beest.
Or something like it.
In the case of including real life animals in their real life form, one could just give them another name. "Mountain lion" is a bit uninspired.
There is no need to make it something completely "unique", just make it
different.