My wife and her sister used to believe that unicorns were real but that seahorses, for some reason, were not. To this day, they still have to think carefully when faced with unicorn/seahorse matters to remember which one does and does not exist.
A childhood belief about general

others believed things like this
- I used to believe that unicorns were real animals until I was about 16...lol...— Tara
- I used to believe that unicorns were real creatures. I thought they were just another type of horse.Anon
- That unicorns were real. They simply had invisible horns that only people with special eyes could see
— Katie - My sister belived in Unicorns until she had 17 years !— ME
- I used to believe that my doll Kimberly was real and if I listened carefully, that I'd hear her speak!— Kristy
- Well,I really think that fairies and witches can be real! We're in a big universe and they can exist!— Raul