The Rancor is a cellar dweller, a giant troll.
This is a totally different proposition to a a Lovecraft style multi-mouth/vagina/tendrill thing.
For one thing the Rancor is upright it moves mostly on a horizontal plane the Sarlacc is anchored to the horizontal plane so has to move vertically.
The other is the scale of the thing.
The Rancor is a mini boss, the Sarlacc is an end of level boss with all the trimmings (including sprouts).
The Rancor is a direct threat only to Luke. The Sarlacc is a direct threat for half the team and death within it would sentence the remaining party to a life of slavery.
The whole sequence is a visual pun.
The dune sea with pirate sailing ships in the sand.
So the Sarlacc should be a Kraken of the sand, a leviathan of the desert.
