Weird West is a literary subgenre that combines elements of the Western with another literary genre, usually horror, occult, fantasy or science fiction.
DC's Weird Western Tales appeared in the early 1970s and the weird Western was further popularized by Joe R. Lansdale who "is best known for his tales of the 'weird west,' a genre mixing splatterpunk with alternate history Western almost entirely defined by the author in the early 1990s. His work reads a little like the sort of folklore in which Mark Twain dabbled (or the Gothic in which Flannery O'Connor was involved), but with zombies and gore."
Examples of these cross-genres include Deadlands (Western/horror),The Wild Wild West and its later film adaptation (Western/steampunk),Jonah Hex (Western/supernatural), BraveStarr (Western/science fiction), and many others.
When supernatural menaces of horror fiction are injected into a Western setting, it creates the horror Western. Writer G. W. Thomas has described how the two combine: "Unlike many other cross-genre tales, the weird Western uses both elements but with very little loss of distinction. The Western setting is decidedly 'Western' and the horror elements are obviously 'horror.'"