Slumpjaw

Also called bog monsters or Larders. They are a manifestation of excess magic. When there is too much unconstrained magic in an area, it attaches itself to a corpse, in this case of Slumpjaws, the skeleton of a fish. The creature will protect itself by taking biomass, mud and other organic things it can find to build out its body and protect the skeleton inside, as this is where the magic is contained.

The easiest way to kill these creatures is to destroy the skeleton, this will make the mass of its body slump away and collapse.