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 Lone Ranger and Tonto are in pursuit of Butch Cavendish when they are fired upon by Cavendish himself, and though he missed the Ranger, he shot and killed his horse. When the Lone Ranger brings up the horse and where it lives, he declares they'll be on the lookout for the horse while they continue their pursuit of Cavendish.
They soon find the white horse engaged in death battle with a buffalo, who has already wounded the horse and is about to finish it off when it is shot and killed by the Lone Ranger. The Masked Man and Tonto nurse the horse back to health, and though he badly wants the horse for his own, the Lone Ranger also understands that the horse has fought for his freedom and deserves to be free. When Tonto remarks that the horse, it's coat glistening in the sun, looks "silver white", the Lone Ranger calls out to him using the name "Silver", and for reasons stronger than gratitude, the horse decides to stay on with him. This started a new adventure for both the horse and Ranger.
The Lone Ranger spent several days training the mustang before he is ready to continue the pursuit of Butch Cavendish. With the new powerful horse as his ally, the Lone Ranger easily catches up to and captures Cavendish.
As with the origin of the Lone Ranger himself, the back story of how Silver came to be with the Lone Ranger was retconned. The Lone Ranger's original horse was a chesnut mare named Dusty, who was shot and killed by Butch Cavendish as 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 put forth on the radio series.
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.