Up until recently, the iTunes Store maintained two separate lists of iPod games — one specifically for fifth-generation iPods, the other for third-generation iPod nanos, iPod classics and fifth-gen iPods. That division has gone away — now the iPod Games list has been unified.
At first, Apple offered some games specifically for fifth-gen iPods and others that could run on a wider array of iPods. Over time that list has diminished, as those original games have been refit to run on a broader range of iPod devices. Now all of those games run on any game-capable iPod.