According to Jim Henson: The Biography by Brian Jay Jones, George Lucas offered Muppets creator Jim Henson the role of Yoda. Henson declined, suspecting that he wouldn't have enough time for a project as ambitious as Star Wars. Puppeteer Frank Oz got the job instead. But Henson still consulted with the Star Wars crew in building the Yoda puppet. You can see more photos of him working on The Empire Strikes Back at The Making of Star Wars.
-via Landa Calrissian