As originally presented, in the December 7, 1938, radio broadcast, Reid had already been well-established as the Lone Ranger when he met Tonto. In that episode, "Cactus Pete," a friend of the Lone Ranger tells the story of how the masked man and Tonto first met. According to that tale, Tonto had been caught in the explosion when two men dynamited a gold mine they were working. One of the men wanted to kill the wounded Tonto, but the Lone Ranger arrived on the scene, and made him administer first aid. The man subsequently decided to keep Tonto around, intending to make him the fall guy when he would later murder his partner. The Lone Ranger foiled both the attempted murder and the attempted framing of Tonto. No reason was given in the episode as to why Tonto chose to travel with the Lone Ranger, rather than continue about his business. A reasonable assumption would be that he felt a sense of gratitude to the man.
A different version was given, with very few differences, in both later episodes of the radio drama and at the beginning of the Lone Ranger television series. Tonto rescues a man named Reid, the sole surviving Texas Ranger of a party who was tricked into an ambush by the outlaw Butch Cavendish (although later reference works referred to the future Lone Ranger as "John" Reid, no first name was ever given to the Lone Ranger in either the radio or TV series). Tonto recognizes the ranger as someone who had saved him when they were both boys. He refers to him by the title "ke-mo sah-bee," explaining that the phrase means "faithful friend" in the language of his tribe. Tonto helps Reid give a decent burial to the other rangers.
The radio series identified Tonto as a chief's son in the Potawatomi nation. His name translates as wild one in his own language. For the most part, the Potawatomi did not live in the Southwestern states, and their cultural costume is different from that worn by Tonto. The choice to make Tonto a Potawatomi seems to come from station owner George Trendle's youth in Michigan. This is the traditional territory of the Potawatomi, and many local institutions use Potawatomi names.
Tonto's name, according to an NPR story on the Lone Ranger, was inspired by the name of Tonto Basin, Arizona.
For more on the subject, see TV Shows Featuring Indians.