The head in question was a bat head finial along the queue just before you enter the doombuggies. It was located at the final turn and was worn down due to countless hands 'sweeping' over it as they made the turn. It then took on a life of it's own, so to speak, and became an incredibly popular icon all HM fans would see out, as everyone wanted to 'rub it for good luck'.
However, it was removed when the HM underwent a rehab a year or so ago and is currently 'on tour', in a way. I was lucky enough to give it one last rub when it was at Henry Ford Museum in Michigan last year (and I happened to be there at the same time).
How do I know this? Research and personal experience. One dealing with the facts while in the HM, the other because I saw the bat in person, a wonderful stroke of luck.