Wiki says....

'The figure has been the subject of much study and speculation, but its origin and age are unclear. It is often thought of as an ancient construction, though the earliest mention of it dates to the late 17th century. Early antiquarians associated it, on little evidence, with a Saxon deity, while other scholars sought to identify it with a Celtic British figure or the Roman Hercules, or some syncretization of the two. Archaeological evidence that parts of the drawing have been lost over time strengthen the Hercules identification. However the lack of earlier descriptions leads modern scholars to conclude that it may date from the 17th century, and perhaps originated as political satire'.

If the last sentence is correct then its hardly an ancient monument is it! You do have to wonder why it was not noted earlier than the late 17th century which was only yesterday in terms of time!