I don't remember how young I was but...
Cats were girls (male) and dogs were boys (female)
Cows were girls and horses were boys

Believing this though never caused a problem in my mind knowing that our family dog was female.

I was really young, maybe 4ish, but I knew that if I didn't have regular meetings with my stuffed toys they would rebel. So we met on my bed weekly and they were allowed to air their grievances.