Doesn't this just come back to a flavor of superstition? Mine were imaginary enemies. I didn't have any imaginary friends, rather, monsters in closets and under the bed or in dark rooms but I clearly remember talking to one when I was 3 or 4 trying to be polite to it so it wouldn't drink my blood. It would also make more sense people have more imaginary enemies than friendly too right? We know that fear really hampers our intellectual faculties and I had learned through evolution we have confirmation bias in that regard as well.