Did the Rancor follow Luke from the
Dagobah swamp? The creature we only see the back of in The Empire Strikes Back.
shareNo, that was a different monster. The fandom wiki calls it a dragonsnake: https://starwars.fandom.com/wiki/Dragonsnake
In the SNES game of Empire Strikes Back, the boss of the Dagobah levels is Habogad (Dagobah backwards), and I think that it's meant to be the dragonsnake. https://starwars.fandom.com/wiki/Habogad
Truthfully, we never really see what the thing is in the film, as you pointed out, but given that it lurks under water, it seems unlikely that it's the rancor, which has no visible advantages for swimming.