For me, it wasn't actually a monster under the bed, it was a fear that the Elephant Man was sitting on the commode in my bathroom. Not using it, just sitting there, waiting to pounce. For years, I was afraid that the Elephant Man was in my bathroom at night. My parents took me to the movie, which came out in 1980, and I had nightmares about the Elephant Man for YEARS.
Now, I fully realize that there was nothing scary about that movie at all. In fact, I'm sure it was just sad. But my five-year-old self didn't know that.
I've considered watching the movie as an adult, but I'm a little worried that the nightmares will come back.