My younger brothers and sister used to believe that dogs ate cats and they were mortal enimies of each other. (at least i finaly explained to them that dogs did not eat cats and that they could be the best of friends).
A childhood belief about cats and dogs

others believed things like this
- My sister and I thought cats and dogs were the same animal, the dogs were male and the cats were female.— Paul
- I used to believe all dogs were male and all cats were female. I still call dogs "boy" and cats "girl." — Kelly F.
- I use to believe that cats and dogs were the same species; cats were girls and dogs were boys.— poopcrystals
- My sister's friend thought cats were just small dogs when she was younger.— Richard Davies
- I used to believe that dogs ate cats.— Tom
- I used to believe dogs ate cats— LEADY