A creature exhibits power to change shape, unpredictably and often uncontrollably. When shapeshifting isn't exotic enough to explain this, Hollywood Evolution or Magic Genetics might do just fine. Suggest that the creature has an Unstable Genetic Code, and it can "evolve" without regard to Evolutionary Levels.
(Note that in these stories, a change on the genetic level always means an (almost) [...] [tvtropes.org]