Dark Souls III Xbox One pre-orders will include a free copy of the original game, and will see the release of two DLC packs, according to the US and Australian Xbox stores.
A US Xbox store listing states the 2011 original Dark Souls is being made backward compatible with the Xbox One, and will ship to pre-orderers on that platform (though separately to the main game).
Presumably, existing [i]Dark Souls[i] Xbox 360 owners will also be able to play From Software's classic on their Xbox Ones soon.