I don't know if this techincally falls under the imaginary friend category, but I supose it will do. When my brother (4 years), sister (5 years), and I (6 years) were little we didn't have imaginary friends. We had imaginary enemies. Their names were Mean Fat Barney and Sally Breaka. They used to always cheat at games and sports. They would also mess up our rooms and throw our toys all over. We would get mad at them tell them to stop but then they would fight us. So we would beat them up and they would go away until another day. Our mom didn't believe that our imaginary ememies were messing up our rooms though, we still had to clean them.
A childhood belief about imaginary friends

others believed things like this
- I used to have an imaginary friend named Mr. Nobody. He would protect me from my imaginary enemy, Bad Elmer.— Kdees
- My little sister's imaginary friend was Kit the talking car from that show Knight Rider.Anon
- my aunt used to have an imaginary friend who instead of having blood had 7-up in her veins.Anon
- My brother was terrorised by an imaginary charecter called anorak doo-dah— Corin
- When I was about 6 or 7, I had an imaginary friend, who left for ice-cream one night. He never returned.— His name was Jimmy
- I used to believe in my imaginary friend dooodle.
Doodle was a stickperson and was always smiling.— Samantha