Chewbacca didn’t spring to life out of nowhere, fully formed when Lucas saw his dog in the passenger seat of his car. That’s the soundbite. A single step. The reality is complex and human. From vague names floating around, the kernel of an idea, changing purposes and roles of characters, major restructuring, the design hopping from person to person, scrapping the existing concept and going down a different path, seeing existing things in a different light and having to conform a range of ideas to complement and enrich one another.
Michael Heilemann has written a superb essay chronicling the evolution of Chewbacca. It's a fascinating glimpse of the creative process, and the way those who participated misremember that process.