Oh man, I could fill this whole topic up with enemies from a Diablo II mod I play, but I'll just list the *really* annoying ones:
Skeleton Knights near the end-game that cast a curse on you, which lowers all of your stats by about 90%. Since all of your gear either has strength or dexterity requirements, this usually results in all of your gear being unequipped, which is probably the most dangerous situation you could be in.
Creatures called "Fireblood" near the end-game that eat the corpses of other enemies and spit huge fireballs at you. If you are hit by one of these, you will almost certainly die.
At the end-game, there are enemies called Steel-Crushers which charge at your character when they see you. If they hit you, they bring your HP down to 1, and you cannot heal your character for about 20 seconds afterwards. I am dead serious on this one. Don't expect to be able to handle them as a melee character.
At the end-game, there are giant magma golems that shoot giant streams of fireballs at your character. If you're caught in one of these streams, you will be constantly stunned and damaged, so you won't be able to move and will eventually die if you don't kill it some other way, or get lucky and somehow make it out of the stream.
And this is my all time personal favorite of them all. In the very last fight, there are tons of blue golems which the final boss summons. When you kill one of these golems, their corpse acts as a sort of booby trap. If you step on it, you will receive a curse that will cause you to die instantly when struck by another enemy. So although there are tons of these things on the screen, fighting them in pairs is a big no-no.
Yes, as you can see, me and my friend have a looooooot of fun with this game