User blog comment:Jackasaurus/Red Dead GOTY/@comment-200.54.151.149-20120111175701/@comment-3455248-20120112065234

The GOTY edition includes all of the released DLC. Since most of the DLC either contained multiplayer content or a separate story (Undead Nightmare), they wouldn't generally affect the single player campaign. The exceptions are the War Horse, as you noted, the Golden Guns pack, and the availability of the Deadly Assassin Outfit. The Hunting and Trading Outfits were also DLC, but free so most people would have experienced them in the game. The Tomahawk and Explosive Rifle were also added by DLC.

If you're on Xbox 360, then you also get access to the Solomon's Folly gang hideout and the Walton's Gang Outfit, which otherwise were only available on PS3.

Finally, the Hardcore single player Difficulty mode is only available with the GOTY edition.