A giant sea worm.
Its completely circular mouth (ringed with more rows of teeth than you can count) opens and closes like a lamprey.
Riders set themselves in the openings along the inner wall of the throat during the ride.
Those used to be saliva glands of the Gordumrunra. Grippli shamem removed them when it was tamed by Mik-Miks great-grandfather, many years ago.
If riders don’t hide in the alcoves the serpent swallowing everything in the central corridor of the creature’s body is washed backward, into darkness – and presumably, the creature’s digestive tract.
The creature does not appear to need any sort of piloting;
and can move at terrific speed through the swamp.