I used to believe that a ferret was the hybrid of a cat and a hamster. I thought the reason they were so rare is because it's hard to breed a cat and hamster without them attacking each other.
A childhood belief about general

others believed things like this
- When I first heard of Amsterdam I thought it was "Hamster's Den"— Bootlebat
- I used to think that if you were the teacher's pet, you would turn into a hamster.Anon
- For the longest time I believed New Hampshire was New Hamster and that was where all the hamsters came from. — Giny
- I used to think an Afghan hound was a breed of terrorist dog that had to fight in the Taliban!Anon
- I used to think that a sea lion was a hybrid of a seal and a lion.Anon
- I used to think that blue was a natural hair color but it as very rare, even rarer than ginger.Anon