If you mean the mouth in the Dune Sea on Tatooine in Return of the Jedi, it is called a Sarlacc. They also live on other planets throughout the galaxy, including Dathomir and Felucia. If you mean the big bleached skeleton the C-3PO walks past in A New Hope, it is a Krayt Dragon, the mighty Tatooinian creature that fallen Jedi Hett names himself after, Darth Krayt.