The giant baby from Zombies Ate My Neighbors.

I can't believe I forgot about Scorpius from Sonic Spinball!  Me and my sister were terrified of this boss when we were younger!

The Exorcist-inspired monster from Gex.

Almost all the villains in the Dreamland stage from the Simpsons arcade game.

That huge snake from the very first Resident Evil game. I think his name is "Yawn".

 

There's some baddies from the Earthworm Jim I'd like to discuss:

In the original, there's a level where it's almost completely dark and you have to find the exits using the spotlights to guide you. Anyway, there some villains where all you can see is their evil, angry eyes. You can kill them, though. But the real, frightening villian is the one with the huge, evil,  angry eyes! And you CAN'T kill him! You have to run away from him!

Another one I want to point out is Jim's dog friend Peter Puppy. On the Puppy Love stages in EJ2, you have to keep the puppies from landing on the ground and into the funnel Peter's holding. And you have to keep them from going SPLAT! It's not easy, 'cause Jim's nemesis Psycrow will throw a bunch of puppies at once making the task VERY difficult. If too many pups go SPLAT, then Peter goes from a good, faithful dog to a mean, ugly, vicious monster that will beat the crap out of you and take away a huge chunk of your energy! Don't let Peter and those little doggies down!

One last one I wanna mention, it's the boss at the end of Level Ate in EJ2. And it's a steak boss. That's right. A steak. It isn't ginormous or anything, but hey, it's a frickin' steak. Once you see this thing, you might stop eating steak lol