I used to think that the penis got its name because you pee out of it, so I thought that by that same token your but should be called your poonis, because you poo out of it. This is a highly common childhood belief.
A childhood belief about weeing and pooing

others believed things like this
- I used to belief sloths lived in trees outside my childhood home. My older sister told me so.Anon
- I thought common as muck was a substance called commoners muck— oddsox
- I used to believe that a penis was called a bobby because my mum told me... I believed this until I was 15.— Patricia
- I thought that the show Beavis and Butthead was called Penis and Butthead.— Porky's Butthole
- I used to believe that the dust at the top of doors was highly poisonous and could kill you.Anon
- I used to think that Frankie Avalon sang, "Hey Penis, Oh, penis..." instead of "Hey Venus."Anon