Chip N Dale's Rescue Rangers debuted on The Disney Channel (and then in Syndication) in 1989, but the characters started appearing in magazines in 1988. I saved this page from a September 1988 issue of Cabletime magazine. The Rangers look a bit different here, especially Gadget the mouse and Zipper the Fly.
Gadget originally had more mouse-like proportions, looking more like Bernard and Bianca from The Rescuers. And Zipper looks a bit more like Evinrude the Dragonfly from the same film.
The original pitch for Rescue Rangers did not star Chip N Dale. There were other mouse characters, including a prototype Monterey Jack and Gadget.
Early promotional artwork like this also appeared in The Disney Channel Magazine prior to the sneak preview of the series.
When The Disney Channel started airing the show again as part of "The Block Party" circa 1995, the Channel's website posted publicity artwork featuring the early designs.
When Rescue Rangers is inevitably rebooted like DuckTales, it will be interesting to see what form the characters take.