Super Mario Odyssey has certainly been buzzing around the internet lately, but did you know that there's been a game of the same name previously!? Yes, it was a hack made (visibly) in 2003 by (visibly) Rusted Rom Hacks. Which hacks Super Mario World to give a demo of a fake game called:
"One year after the events of Super Mario World, Mario and his friends have taken a vacation to a new island. They soon learn of Bowser's presence on the island, and find out that he has stolen Yoshi eggs again.Not to be confused with the newly announced game "Super Mario Odyssey, this hack has seemingly no other connection to the new game, than the title it'self. I found this while browsing the web for images of the new Mario game. It still shocks me to see the title is the same, besides the subtitle.
Or so it seems. After Mario journeys through the Alto Plains for a while, Bowser's new Nightmare Base appears, and Mario explores it. Yoshi hides in a storage area, and contacts Mario remotely. He tells Mario that Bowser has learned how to time travel by using the Spheres of Light and plans to send the Nightmare Base into the past so that he can change something, presumably something involving Peach or the Mushroom Kingdom.
After fighting through the base for a while, Mario encounters a warp that sends him into the past. Here, he travels through the Village of the Sun, through the Candy Hills and up a steep cliff, and eventually finds the Village of the Moon. The inhabitants inform him that a Sphere of Light is hidden inside the Twilight Temple. Mario journeys to the Twilight Temple, defeats Stony XVII, and claims the Sphere of Light.
Mario returns to Bowser's base and reaches the cooling system section. The demo ends after this area."~(SOURCE)