Random Vampires (Most Common)
Random Dawnguard (If Vampire)
Random Werewolves (Seem to be rare)
Dawnguard Member VS Two Vampires. (The Dawnguard guy will drop his gear after the Vampires are dead due to "I quit! I can't do this!")
Two Dead Skeletons.. If you walk up to them, a Vampire Assassin will teleport and attack you.
If you have one of the Aetherium pieces, you may find a guy who will ask where you got it. After explaining, he and his guards will attack you.