As a child I was terrified of yetis (despite the fact I live in England and they don't) and I was certain that they lived at the bottom of my garden and in the cupboard under the stairs/other random places in my house. Whenever I went down the garden I would freak out and run back up it, screaming.
This belief was shared by my twin sister, as was the idea that, if you flushed the toilet, the yetis would come running (though I have no idea why they would do that or what they'd do when they got there).
A childhood belief about around the house

others believed things like this
- I used to believe that elfs lived in the hedge at the bottom of my garden!!
— The milky bargirl - Looking back at it, the belief that dogs eat cats is a pretty funny idea. I used to believe it myself.Anon
- There was a custard well in my back gardenAnon
- I don't know why. But I always ran away screaming after the toilet flushed.— Bijan
- I used to believe that monsters lived in my toilet and they ate my poop and urine when i flushed the toilet.Anon
- As a child, my twin sister and I used to pronounce `Spaghetti' as `Mastaghetti'. Sounds a bit rude, no? Anon