When I was very small, I thought everyone was female and when I met a boy at the kindergarden, I thought he was a girl too, and called him a girl. I think it made him cry.
When I was about 4 or 5, I thought nobody could run faster than me.
Around the same time, I also believed that what you see on TV is really stored inside the TV and if you broke open the TV, you could take all that stuff out. I really wanted to do it one time when I grow up. I also believed that everything (our daily lives and our neighbors’ lives, everything) was being filmed and shown somewhere on TV and some other ordinary people, somewhere, were watching all that.
I also believed I would die before my parents would die (I thought this in a positive way) and my mom would play with my bones for fun.
When I was about 7, I read a newspaper article about scientists working on creating some superhumans, which would be vastly more powerful than us and would easily kill us if they wanted to. It filled me with dread for about a year.
Around the same time, I pretended I had a homeless twin sister and sometimes pretended to be her.
When I was 9, I pretended I had more than one personality and almost kinda believed in that, but it was more for fun. Some of those personalities were anime characters, so you can say I was almost an otaku kin.