Horror and Western seem like two genres that would rarely, if ever, mix, but in fact, cinematic horror-Western hybrids date back to the early silent era of film. Here's a not-quite-comprehensive list. (For the sake of this list, I'm sticking with traditional Westerns -- you know, cowboys, horses and such.)
Haunted Range (1926)
This early John Wayne feature stars the Duke as a man who inherits an abandoned mine that's rumored to be haunted. The "ghost," however, turns out to be more of the Scooby Doo "I would've gotten away with it if it weren't for you darn kids" variety -- as was the case in several ghostly westerns of this era, including Speeding Hoofs (1927) and Haunted Ranch (1943).
Curse of the Undead (1959)
The Living Coffin (1959)
In this Mexican adaptation of the Edgar Allan Poe story The Premature Burial (adapted three years later to greater acclaim by Roger Corman), a cowboy lawman visits a family haunted by ghosts and vampires. This is one of several Mexican horror westerns of the era, including The Headless Rider (1957), Swamp of the Lost Monster (1957), Night Riders (1959) and Ship of the Monsters (1960).
One of two ridiculous camp horror-Westerns (see below) from low-budget "quickie" director William "One Shot" Beaudine, this film places Dracula in the Wild West, plotting to hypnotize and marry a local young lady. However, her fiance is none other than Billy the Kid, who vows to put a stop to the vampire shenanigans.
A Civil War soldier returns from the dead to take revenge on the officers who betrayed his unit in battle. One of dozens of (often odd and explicitly violent) unofficial sequels to the 1966 "spaghetti Western" Django, Django the Bastard (AKA The Strangers Gundown) is rumored to have inspired Clint Eastwood's High Plains Drifter (see below).