My thought was it's sort of a mutated sea anemone, plus a bit of lamprey and humanoid. But you do make a fair point that it could be more accurately called a monster rather than a mutant. The prompt for the image was made using a random "mutant" generator, which is why I called it one.

You call these creatures mutants, but what are they mutated versions of? Their shapes suggest humans, but the degree of "mutation" suggests that they're something else, with the mutation being the vaguely humanoid shape.