When I was little, I believed in something I called 'the bathroom genie'. He looked like the little blue dude from Aladdin, but he lived right at the bottom of the toilet (rather than, say, the bottom of a lamp). If you didn't get off the toilet immediately after going to the bathroom, he would come out and eat you. This troubled me in the hygiene department for a few years.
A childhood belief about fear of

others believed things like this
- i used to be scared to use the bathroom becuase i believed something lived in the toilet.— robert allen freeman the fourth
- I used to believe that monsters lived in my toilet and they ate my poop and urine when i flushed the toilet.Anon
- I used to believe that elfs lived in the hedge at the bottom of my garden!!
— The milky bargirl - I used to believe that your bottom teeth were actually in your bottom— jez
- David Bowie - Jean Genie
"The Jean Genie lives on his back
The Jean Genie loves cheesy snacks"— Santa Puncher - I believed that fairies lived in lamp posts - their front doors were the little holes where the key fits.— Janice