Underwater action game Steel Diver was originally going to be a Nintendo DS game, but it's since surfaced on the 3DS.
According to Mario creator Shigeru Miyamoto, the game was "well along" in its development when it was decided that the game should be a 3DS title instead.
In fact, development on the game was pretty much finished, and the game's developers were in the process of debugging the title.
It isn't unheard of for Miyamoto to shift gears on in-development games (or even outright can them). According to Q-Games' Dylan Cuthbert, Miyamoto isn't thinking about money; he just wants to make the best game when he upends the tea table.
Steel Diver was originally a tech demo to showcase the tech capabilities of the Nintendo DS. This is a game years in the making.