I used to think that a gelding was a breed of horse.
And I used to think that geldings and mares couldn't be turned out together. And I used to not be able to tell the difference between a Friesian and an Andalusian. And I used to think crops and spurs where abusive. Hehe...
Oooh! Just remembered another one (looking at your avatar ^^). I thought that Arabs were the only horses that lifted their tails up. So everytime I saw a horse trotting, cantering or galloping with their tails up I assumed it was an Arab.
And I also thought that a thoroughbred was a term used for any pure bred horse.
When I was little I really wanted a horse but we didn't have enough room. So, I was convinced that I could keep a miniature horse in the backyard instead. I had it all worked out! It could sleep in the (tiny) supplies shed at night and I would take it for walks around the streets like a dog. And people would think I was the coolest person on the block!
-_- Thank god my parents never let me do that, haha.