According to the Australian Xbox Store, players that pre-order the digital version the Dark Souls 3 will receive a free copy of the original Dark Souls. As of the time of writing, the page has already been removed.
The description said that download codes for Dark Souls would be sent to pre-order customers 7-10 days after purchase. This means that the original Dark Souls could become backward compatible with the Xbox One in the next few days. Microsoft and Namco have yet to make an official announcement about this situation.
Dark Souls 3 releases worldwide on PS4, Xbox One and PC on the 12th of April.