From Monkey Island wiki
The Mad Monkey is a huge galleon that sank prior to the events of the Monkey Island series. It was reputed to have a figurehead that was the most fabulous piece of art ever, so that collectors such as the Antique Dealer on Booty Island are willing to pay up to six million pieces of eight for it.
Though the antique dealer believes that nobody knew where the ship sank or why, the book Great Shipwrecks of Our Century (which can be checked out from the Phatt City Library) has the location of the wreck. The figurehead, when Guybrush finally discovers it, turns out to be a giant brass monkey head.