People who don't live with rabbits often don't realise they have proper tails. Even from fairly close, most bunnies look like they have a ball of floof stuck to their butt, but what you are seeing is just the end of the tail. Like most animals, our little fluffy friends have proper tails they use to aid their balance, it's just that theirs are mostly tucked underneath out of sight
At the Rescue, we are of course well aware of the true shape of a rabbits tail because we deal with so many sick and mistreated bunnies needing them trimmed or cleaned. When the tail becomes wet and the floof flattens down, you see an entirely taily-looking tail.
For all my experience and time around rabbits though, I must admit I don't see this all that often - as Einstein relaxed into his dreamy mega-afternoon-nap, his big floofy tail just rolled out flat behind him! How cool (and slightly weird looking) is that??!!
(Note - keen-eyed readers will have spotted that this is the other end of what we saw yesterday.)