There are many theories about how the Giant got there. Some say he's a Roman god. Some say he's a monkish joke. I would point out that the first reports of him are in the 17th Century. So I have two theories of my own.
One is that he's original and old. In which case he is, almost uniquely in English archaeology, a genuine fertility symbol. There's just something - maybe it's the jaunty angle of his club, I guess - which suggests that.
The other possibility is that he was the result of a giant piece of 17th Century graffiti. I like to think that on the downs next door it said "William Stukeley woz ere", and at the bottom of the valley somebody had drawn the words "Welcome to the Hood".
