This is not a toilet beleif, but similar. I used to refuse to let the bathwater out of the tub because I was conviced that the drain was a tube that ran straight down and ended in a room and the water poured out on a lion's head and woke him up. I thought one day he would stop roaring at me and just break through the bottom of the tub and eat me.
A childhood belief about bathrooms

others believed things like this
- I asked my father what holy water was. He told me it was the priest's bathwater. — Gwen
- if you eat toothpaste straight from the tube, you will get worms— wormelow
- You must break the bottom of egg shells after finishing a boiled egg or witches will use them for boatsAnon
- I used to want hemmoriods so I could use a "float tube" in the tub and have a cushy toilet seat like my dad.— Tami
- how many us believed you would go down the drain with the bath water? I did!— Amy
- I had the weird superstition that if I let all the bathwater run out it'd suck my soul down with it too.— Dan