Cacodemons: They float in the air, belch ball-lightning, and boasts one Hell of a big mouth. You're toast if you get too close to these monstrosities. The first time I played DOOM I walked around a corner and I heard a HIIISSSSS which made me PIIISSSSS my pants!
Barons of Hell: The Hell Knight was bad news, but this is Big Daddy. These bruisers are a lot like Hell Knights, but look a little different and are twice as tough to kill.
The Spider Mastermind: You guess the Arachnotrons had to come from somewhere. Hi, mom. She doesn't have a plasma gun, so thank heaven for small favors. Instead, she has a super-chaingun. Crap.
The Cyberdemon: A missile-launching skyscraper with goat legs. 'Nuff said.
Now I can rest...