Nice thread!

I rarely see any monsters in my dreams anymore. I think I killed them all when I started having lucid dreams. When I have nightmares its usually being afraid of something non-physical.

I remember the monster I was the most afraid as a kid, although it's not really "badass".
It was a recurring dream. The scenery was my own room, but dark and somehow twisted. I got up and looked through the window. The sky was red, and there was a storm. I had a strong sensation of being alone, not only in my house, but in the whole world. Then I felt a presence behind me. I turned back in fear, and there it was... A garden dwarf, smiling, with a fillet knife.