Hi Tim,
It is strange, it seems to appear in the menu once you visit the monsters page, but opther pages it doesn't. In my tests the page is always accessible (so it isn't like the page does not exist) but I do see where it is not correctly shown in the menu depending on what page you are on.
The first thing I would do is change to a different skin and see if it happens also in other skins. If it doesn't happen in other skins then we can assume it is skin related and try to figure out what it is about the skin that is different. Possibly something about the menu configurations or SiteMapDataSource configurations in layout.master.
Hope that helps,
Joe