The Rancor is a giant humanoid monster that is kept under Jabba the Hutts palace room. It is humanoid because it has two arms and walks on two legs but it definately could not walk down the street without someone run screaming from the Rancor. People who displease Jabba are sent via a chute into the Rancors cell and eaten.
In Star Wars VI - Return of the Jedi, the Rancor eats a green female dancer that Jabba, the Hutt had grown tired of. When Jabba tries to feed Luke Skywalker to the Rancor, Luke is able to defend himself and kill the creature, much to the displeasure of the Rancors keeper who cries.