Sarlacc

"In his belly, you will find a new definition of pain and suffering as you are slowly digested over a thousand years."

- C3PO translating Jabba's description of the Sarlaac to Luke Skywalker and Han Solo. The Sarlacc is a gigantic alien monster featured in the Star Wars franchise and became one of the series most iconic and popular aliens as well as one of its more dangerous - it is also one of Star Wars more colossal specimens and was largely hidden due to its underground nature.

Biology
The Sarlacc, as mentioned above, lies submerged under the ground but appears as a gaping maw full of sharp fangs - easily large enough to consume human beings (or larger prey).

Return of the Jedi
One particularly powerful specimen was utilized by Jabba the Hutt as a tool of execution for those who displeased him and he attempted to have Luke Skywalker, Han Solo and Chewbacca sacrificed to the monster but was unsuccessful. It instead ate several of Jabba's guards and Boba Fett during the battle, though the bounty hunter escaped after several agonizing days.

"A Barve Like That"
In J.D. Montgomery's tale "A Barve Like That", Fett awakens to find himself trapped within one of the Sarlacc's chambers, protected by his armor but restrained by a multitude of tentacles. He is ridiculed by a voice claiming to be of the Sarlacc itself, stating that Fett's existence and everything about him belongs to the Sarlacc's sadistic whims. It is revealed that the voice belongs to a young Choi named Susejo who was the Sarlacc's first victim. It absorbed his consciousness and uses it as a speaker, showing a kind of connection between Susejo and the Sarlacc. (For example, when Susejo laughs, the Sarlacc shakes as though it too were laughing.) It is further revealed that the Sarlacc breaks its subjects' minds and absorbs their consciousness.

While entrapped, Susejo/Sarlacc torment Fett with existential taunts and exposing the bounty hunter to the collected memories and consciousness of its victims in hive mind fashion. However, this plays a pivotal role in Fett's escape once he learns of a Jedi woman whom Susejo liked and failed to save from being digested by the beast. Fett criticizes Susejo for doing nothing to save any of the Sarlacc's victims when he has the power. Angered, Susejo directs the Sarlacc to constrict its tentacles around Fett in an attempted strangling/suffocation. The tentacles trigger Fett's jetpack which explodes in the tight quarters, setting the monstrosity alight. With sheer will, burnt skin, and a broken arm, Fett escapes to the surface. He is found by fellow bounty hunter Dengar and nursed back to health.

A year later, Fett passes over in the Slave II and is mentally confronted by Susejo who believes that Fett has come to kill it in revenge. Fett uses the thrusters of the craft to incinerate many of the creature's tentacles. Susejo questions what kind of mercy this is, only for Fett to dismiss Susejo and take off once more.

Other Appearances
Since then the Sarlacc have appeared on and off in varied Star Wars media.