The Nintendo DS version of Unleashed, Double Smash features gameplay akin to a side-scroller, similar to that of the Godzilla: Monster of Monsters. Although graphically 3D, its 2D gameplay could be said to make the game 2.5D, much like in New Super Mario Bros. or Sonic Rush. Using the two-screen display of the Nintendo DS, flying monsters appear on the top screen, while grounded monsters appear on the bottom screen. A multi-player option will allow for a different player to control each monster.
Source: Wikipedia, "Godzilla: Unleashed", available under the CC-BY-SA License.