Part of the deal that brought the Smurfs to NBC in 1981 included another set of characters from the pen of Smurf creator Peyo. Johan & Peewit debuted in Belgian comics written & illustrated by Peyo around the late 50's, and met the Smurfs in a number of adventures, some of which may or may not have been adapted for American television.
Johan was a page in the King's court, and Peewit (pronounced Peewee) was his sidekick and best friend. The duo made their American debut, and their first television meeting with the Smurfs, in the episode, "The Cursed Country".
As you can see, there was an alternate open, using a theme tailored for our heroes, but using the visual open for Smurfs.
Michael Bell, also the voice of Handy Smurf, spoke for Johan, while Frank Welker was the voice of Peewit, whose voice was not too dissimilar to that of Marvin from the original Super Friends. Johan & Peewit became a regular feature on Smurfs, especially after the series was expanded to 90 minutes.
For what it's worth, and I only found out about this recently, the "Smurfs" feature film has had its released pushed up from its originally announced December date to July 29, so you can expect Boomerang, which ran a Smurfs marathon earlier today, to do another one around that time.