I'm not sure where they originated, but when I was little I had a couple fears of the toilet. The first one I think came from partially noticing something playing on tv, but I thought the toilet would wait until I sat down and then suck me in and eat me, with the lid kind of making a mouth.
Another time I was convinced that there was a snake living down the toilet drain that would pop out once I was seated and couldn't see it. Sometimes the snake blended in with the hungry toilet idea and became a kind of tongue.
... Needless to say, I made a habit of going up to my mom and casually asking if she'd used the toilet each day, to make sure it wasn't in a hungry mood and that the snake wasn't around.
A childhood belief about fear of

others believed things like this
- I used to believe that if I sat on the toilet too long a snake would come up and bite me.— Cheesemonkey
- I thought there was a beaver living in the toilet that would bite me if I wasn't ever vigilant. — Camille
- I thought that toilet paper was bread and by puting it it the toilet i was feeding the ducks at the bottem.— Gringo
- 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 if I flushed the toilet before I showered, I would take a shower in dirty toilet water.— Chris D
- I used to beleve when i was 4 that the flush on the toilet was to wake up toilet monsters.— Millie