Skeevers and Wolves, they pop up (like the OP stated) out of nowhere and scare me every time.

Hagravens, they're just plain creepy and... wrong.

Slaughterfish, I can't freakin' see them while they're under the water, nor can I attack them while I'm in the water. My best hope is to just Shout at the water or spam Flames/Frostbite into it. Makes swimming across deep areas a [censored]. :meh:

skeevers they are ninjas.

Ok, after playing today, I have to add ice mages to the list. The b$%tch slowed me down so much with her ice spell and then bang - ragdolled away...happened 2 or 3 times before I finally killed the ho...

The Mages of any stripe are pains.

But beast wise, if I don't have a high spot I can snipe from, Frost trolls are the bane of my life, even at level 30.

this ^

Ancient dragons. I have 90% less mana cost for destruction spells and i still go OOM when fighting those bastards.

Cougars.

Damned older women can't leave my thief alone.

Falmer Skulkers and Hagravens

Hagravens and Falmers.

Ice Wraiths, difficult to see them in snow storms while fighting them.

Deer. Elk. Moose.

I can kill anything else in some manner... but these guys always somehow see me every time and start running. I can catch up to them now, but in early levels... very annoying when trying to get leather.

Funniest thing I've read all day.

Annoying? that would go to bandits. Always its one of four guys. The nord with the big hammer. The redguard woman with the mace, Mr. T, and the breton with the bow.