A rancor is a large reptilian carnivore native to the planet Dathomir.They are about five meters tall,with long arms,horrible claws, armored hides and immense toothy jaws in a pug-nosed face.
Jabba the Hutt kept a rancor in a pit below his palace court, to which he fed any who displeased him.
In Return of the Jedi, the rancor was first tried out as a man in a creature suit, but the results were unsatisfactory. The beast was then made as a rod puppet, and filmed with a high-speed camera. Then its movements were slowed down to give the illusion of a much larger creature, moving a bit more ponderously.