How was he a Lone Ranger, if he had Tonto with him?

Sounds right to me.

Good answer.

TheGreatBambino said: "He was the last remaining one from the group of Texas Rangers that he used to belong to before they were massacred."

You're exactly right, GreatBambino. In fact, that's how the name is introduced in the "origin" episode that tells about the troop of Texas Rangers being ambushed by the Cavendish Gang at Bryant's Gap. When whatever-his-name-used-to-be regains consciousness, Tonto breaks the news to him by saying, "You only one left. You lone Ranger."