From Rockstar's blog post
Earlier today on Rockstar Games website, it was announced that more info about the Undead Nightmare pack will be released through out the course of this week. This info includes a teaser for the pack coming this Thursday, September 30th. Please stay tuned for updates on the pack through the wiki's Undead Nightmare page (click here to go to page) or the RDR official site (http://www.rockstargames.com/reddeadredemption/dlc/undead)
Update:Go check out our related blog post to see in-game screens of The Undead Nightmare pack (Click here)
Update #2:The Trailer has been released! Check it out here