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An expert staff ghost is like any other ghost, the only diference is that they're extremely fast. You can unlock them once you have beaten a certain record on a course. For example, your first ghost race is against any worldwide ghost (choosen randomly) if you beat that ghost you will race against a fast staff ghost and if you beat that ghost you'll race an expert staff ghost. These are made by the ones who created the game, so they're more likely to use shortcuts on the course so be prepared. Oh and a retro course is the one that came from a past game, like N64 mario raceway or DS yoshi falls. And the retro cups are the shell, banana, leaf and lightning cups, as they contain only retro courses. Hope this helps!