My cousin used to believe that a beagle was a type of a bird(from getting it mixed up with eagle)
He would read the Peanuts comic strips, especially the ones with the beagle scouts, which are actually a group of birds led by Snoopy. He had no idea what a beagle was, so he guessed that the "beagle" in Beagle Scouts was referring to Woodstock and the other birds, until he asked his mom "What type of dog is Snoopy supposed to be? I know that Woodstock is a beagle but what about Snoopy?" and he got laughed at and she explained the whole thing to him.
A childhood belief about cats and dogs

others believed things like this
- I used to think the song "Hang on sloopy" was "hang on snoopy"— gina
- I thought birds grew from bird seed. — Jen
- I used to think an avalanche was a type of bird native to the high mountains— Nina
- When I was eight, I thought my dad could translate what the birds were saying. I have no idea why.Anon
- I used to think an evil spirit type thing called Wango lived in my dog. I was terrified of that dog.— Franklin
- i used to think flies were little birds!— unforgiv3n