Yes, exactly - I knew that whereas Dean's imaginary friend (Harley) was a substitute for a sibling, Sam's is a way of modeling Dean's big-brother-behaviour. But of course, he'd just been to the zoo and Sam's the kind of kid who obsesses, I think, about things that interest him, so instead of a boy, he has a lion. Makes perfect sense.

And Sam's also enough of an outside-the-box thinker that I think he wouldn't see any incongruity in having a lion for a "little brother" - imaginary or not.