Even the first couple Rocky's? In Rocky 3 and 4 for sure he was eating out three times a day at the all-you-can-eat Franco Columbu "vitamins" buffet, but if this is not naturally achievable then I don't know what the point to working out is haha:
Actually Stallone started to juice for Rocky, there are some pictures of him a few months out before Rocky, where Stallone looks like shit, seems like he was totally unaware about his fitness back then, he smoked and so on.
He was clearly on in Rocky I and II, although that doesn't mean his body in those movies is not achievable naturally.