Pistol, from Martian Dreams
Apparently a normal, but beautiful black stallion, Pistol was incapable of speech -- unlike his competition in the auction block, which included Smith the talking horse, and Earp's dream-incarnation as a horse named "Marshall." When spoken to, Pistol simply nudged the Avatar's hand looking for sugar cubes or some other treat. Finding none, he turned away.
Pistol was not seen again once Earp's dream was ended.