For the first several seasons of King of the Hill, Dale goes by the alias of Rusty Shackleford when dealing with personal identity matters. Dale has stated he created this alias when a student by the same name died when the boys were younger, thus giving Dale the opportunity to use his name.
In the Season 11 episode Peggy's Gone to Pots, it's discovered that Rusty is indeed still alive and only moved away when the boys were younger. Rusty comes back to Arlen to resolve the numerous identity issues that have risen from Dale using his name. After Dale and Peggy's failed attempt to stage their deaths, Rusty has Dale sign a form clearing him of all his identity troubles. Rusty is never seen or mentioned again for the remainder of the series.
Episodes Dale refers to himself as Rusty Shackleford