Silver is the Lone Ranger's great white stallion.
The accepted story of Silver's origin has the white horse living in Wild Horse Canyon. Sometime after the ambush at Bryant's Gap, the Ranger and Tonto go there to find a new mount for the Ranger. They find the white horse engaged in battle with a buffalo. The horse had been severely injured, and the Lone Ranger kills the buffalo in order to save the horse. The masked man and Tonto nurse the horse back to health. This started a new adventure for both the horse and Ranger.
When the horse has recovered from his wounds, the Lone Ranger prepares to set him free, but Silver, a name suggested by Tonto, decides to stay with the masked man. The Ranger spend several days training the wild horse before he is ready to continue the pursuit of Butch Cavendish.
On the radio show, the Lone Ranger had Silver for a long while before any thought was given to where he came from. Eventually it was told that the Lone Ranger was riding a chesnut mare named Dusty, who was killed by an outlaw he and Tonto were pusuing. After Dusty's death, the Ranger and Tonto proceeded to Wild Horse Canyon and happened upon Silver.
It is revealed in the radio episode, Radio: The Theft of Silver that Silver is shod with silver horseshoes marking him as the most famous horse in the West.
On television, Silver's story followed the accepted legend.
in 1981's The Legend of the Lone Ranger, Silver's story was changed to show John Reid rescuing Silver from a trap rather than a buffalo fight.
In the Dynamite Entertainment comic book series, Silver's origin was also altered. While the horse still had an encounter with a buffalo, John Reid bought him from a horse trader after Silver was almost completely recovered from his injuries. Reid still was about to set the horse free, but Silver decided to stay.