A giant mutated tadpole.
I think it looks like a fish. Only with legs. If you look at it and imagine it without legs, it basically looks like a big, blue goldfish.
shareI also believe it's a gigantic mutated fish.
In one of the creature's last scenes, it spews out a smaller (mutated?) fish, could be a clue to it's descent. Also could have been his dessert of course...
I read elsewhere that the largemouth bass was introduced into the river system and has caused havoc. It's possible that the bass, which can grow to enormous size, could be mutated.
I found this at Wikipedia:
The monster was designed with some specific parameters in mind. According to the director himself the inspiration came from a local article about a deformed fish with an S-shaped spine caught in the Han River. Therefore, the director's wishes were for it to look like an actual mutated fish-like creature, rather than have a more fantastical design. In the opening scenes of the film, two fishermen presumably encounter the creature whilst it is still small enough to fit in one of their cups; suggestive of its humble, more realistic origins. The monster also exhibits frontal limbs similar to amphibians' legs. This element of its design seems to have been more a choice of functionality on the designers' part as the monster needed to be able to run and perform certain acrobatic movements during the film. For a genre film monster, the creature's size is rather small, only about the size of a truck. Also unlike in many other monster-themed films, the creature is fully visible from early on in the film, sometimes for large stretches of time and even in broad daylight, which earned the film some critical praise.
*** The trouble with reality is there is no background music. ***