Last year, Microsoft hosted a vote to see what Xbox 360 titles folks wanted to see on the Xbox One via backwards compatibility. Rockstar's Red Dead Redemption came in third place behind Skyrim and Call of Duty: Black Ops II. We haven't heard anything about the glorious ol' Western coming to the Xbox One, but that didn't stop it from stealthily appearing on Microsoft's store.
LyleLanley on NeoGAF found that if you own a digital copy of Red Dead Redemption on Xbox 360, you can download it on the Xbox One. But! In order to do so, you have to follow "rockohoward" on the store, view his profile, view what he follows, and then choose the game (thanks Yabo!). Unfortunately, if you have the disc, you won't be able to use it for now.
This all reeks of a strange accident, and I hope we get official word from Microsoft soon. Since you can't play the DLC through this method, and Undead Nightmare is one of my favorite expansions ever, I'll be keeping an ear to the ground. Let us know if you're able to download the game!