The animated series introduced a three-armed, three-legged alien member of the bridge crew with a long neck named Arex and a cat-like alien crew member named M'Ress. According to Roddenberry, budget limitations would have made it impossible for either alien species to appear in a live action series of the time. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.