The Arlings used to live on the Range as well but then left it together with the Creators. What we do remember about them is that appearance-wise they look almost the same as humans. But they were certainly different, having two hearts, being able to see in ultra-violated and infra-red diapasons and to grow back their limbs in case if they lost one.
Unlike the Creators the Arlings were mainly concentrating their effort on bio-technologies and have made a lot of progress in this respect. Together with the humans the Arling formed a very strong alliance.