The present example is a rare Roman depiction of the hero, not known from any other surviving examples in the round, but surely based on or inspired by a Greek original of the Late Classical Period. As Boardman informs (p. 792, "Herakles" in LIMC), "The 4th century witnessed the creation of the greatest number of sculptural types of Herakles, many of them copied and adapted in later periods."
Closest to our marble in terms of the treatment of the muscular body is the Albertini Herakles in the Museo Nazionale, Rome (no. 288 in Boardman, op. cit.), but here the lionskin is draped over the left arm rather than the shoulder. The Albertini type is known in several surviving versions, and Boardman informs (op. cit., p. 792) that it "can be dated around 385 B.C. It is attested almost simultaneously in South Italy and Attica."
Closer still to our marble is a depiction of a statue of Herakles on an Apulian red-figured column-krater in the Metropolitan Museum of Art (no. 271 in Boardman, op. cit.) where a sculptor attends to the finishing touches by applying color to the lionskin. Here the skin is worn over the shoulder, but the left arm clutches the drapery, the hand emerging. On the vase the hero is extending his right arm, the hand resting on his club, which is supported on a separate plinth. Despite the differences in the treatment of the lionskin, this is likely the original pose of our marble. The hero may also hold a bow with arrows, or a drinking cup, and less commonly, a phiale, cornucopia, apple, laurel branch or quiver.