Auctioneer, from Martian Dreams
The Auctioneer was a fabrication of the mind of Wyatt Earp during his nightmare in Martian Dreams. This fast-talking gentleman moderated the auction between the Avatar and Ike Clanton for Earp, in his dream-guise of "Marshall," a charcoal-grey horse. Also available for auction were a beautiful (but speechless) black stallion named Pistol, as well as the infamous talking brown horse known as Smith.
Once the Avatar had won the auction, the Auctioneer vanished, leaving Wyatt Earp to stew over the fact that he'd ended up nearly being sold to Ike Clanton.