Wookiee
Wookiees are large hairy humanoids native to Kashyyyk. Wookiees rarely wear clothing and are incapable of speaking "Basic" (the common language of the Republic and Empire), but they are quite intelligent. The Wookiees were enslaved following the rise of the Empire. Many Wookies were skilled contractors, engineers and navigators. The Empire enslaved wookiees to serve as workers in various engineering projects.
It was noted in the book Death Star that enslaving Wookiees was unnecessary. In many cases, they would have worked on the projects of their own accord for the technical challenges.
Wookiee Language
Wookiees speak in what humans might regard as a roar or moan. They appear to be unable to produce the phonemes of humans, although they can understand them easily enough. Likewise, humans and other species who can understand Wookiee don't seem particularly rare (Han Solo, Lando Calrissian, Rey, Yoda, and a medic in the Resistance base all understand Chewbacca), but other species don't seem to be able to speak Wookiee.