In the scene where C-3PO walks through the dungeon corridor a creature puts its clawed hand through the bars of its cell, startling C-3PO.
Is this creature ever identified, perhaps in the Expanded Universe (Legends)?
The novelisation isn't much help
Periodically a hand or claw or tentacle would reach through the bars of a door to grab at the hapless robots.
Nor is the film's junior novel
“What could possibly have come over Master Luke?” C-3PO wondered aloud. “Is it something I did? He never expressed any unhappiness with my work.” C-3PO saw a repulsive hand reach out between the bars of a cell door and try to grab him. “Oh! Oh!” the protocol droid exclaimed. “How horrid!” Trying to avoid the hand, he moved to the other side of the passage.
The script isn't any use
One of Jabba's Gamorrean guards marches Artoo and Threepio down a dank, shadowy passageway lined with holding cells. The cries of unspeakable creatures bounce off the cold stone walls. Occasionally a repulsive arm or tentacle grabs through the bars at the hapless droids. Artoo beeps pitifully.
It doesn't even get a mention in the new junior novel
"What could possibly have come over Master Luke?” chatters C-3PO. “Is it something I did? He never expressed any unhappiness with my work. Oh! How horrid! Ohh!”
A tentacle has reached out from a filthy cell they are passing and wrapped itself around C-3PO’s neck.
The Complete Locations book merely refers to it as a one of several "Captive[s] awaiting torture".