Rayman 2: The Great Escape.
I loved this game as a kid, and I still do. But one thing I hated about it: those black caterpillars. Hairy, black, disgusting crawlers...and the noise they were making didn't help either. They seriously gave me nightmares.
Once, my game crashed just after I got through a whole cave full of those beasts, and I had to do it again. Pure horror D: