Yeah. One is an alien creature you do not know the capabilties of, the other is a common wild animal that shouldn't be up to the challenge of an armed humanoid. And that's ignoring the issue of creativity.
Well, yeah, there's that. If you go with established wildlife, players will have some inkling of knowledge of which ones are the most powerful, before getting into a fight with all of them. Which, if you're designing a game, could sometimes be something you might actually WANT to happen. And while the alien wildlife certainly lent to the general alien feel of the whole province, I'm really not convinced that it's always better to have huge insects instead of mammals just because.
Nobody's saying that it's
always desirable, but it can be if originality is what you're after. Oblivion, for example, was purposely designed to be as generic as possible, and we all know how that turned out. As for knowing how powerful your enemy is, if the game designer isn't a blithering idiot that's not really a problem. Take a look at
this and
this. Which do you think is more dangerous? Yeah, that'd be the one whose face is pretty much made up of nothing but sharp teeth as long as your arm.
Well, you had those weird egg laying creatures. Called skooma or skoomoo or something like that. They were fun ^^.
Kwama. Skooma is a drug made out of Moon Sugar. Most merchants won't even talk to you if you have that stuff in your inventory, but Khajiits love it.
And yes, I know that off the top of my head.