Rubber Duckies, amphibious apex predators

[Prompt: A species known for its mischievous personality]
For rubber duckies, "known for their mischievous personality" is putting it light as feathers. (super)Natural predators of both iris beetles and skitters, these trepizone-dwelling critters are capable of transforming into huge, horrifying monsters, and ruining minds and lives. It's mischief, all right, if "mischief" can also ecompass life-altering trauma.
At first glance, rubber ducks seem comically harmless. They bear a strong resemblance to earthen, manufactured rubber ducks, and some have speculated these toys somehow played a role in duckies' evolution.
(Unknown to the world's characters, these rubber duckies ARE rubber toys from Earth. Once, a transport ship (freighter?) spilled a massive number of rubber ducks into the ocean, where they scattered to the waves. Over time, a lot of the toys found themselves in the paths of currents, and subsequently aided in the study of ocean currents. Somewhere along one of these currents was a portal to Olua, where, with a combination of magic & bullshit, they gained something like life. They do the non-reproducing immortal equivalent to evolution to this season.)
But they have a trick up their stretchy sleeve. When hunting, in need, or simply in the mood to prank, rubber duckies can transform into huge, toothy, spiny dragons. Wyvern might be the closest analogue to what these guys become; upon trasformation, they gain both claws and powered flight in their forelimbs. In a "final form" kind dynamic, duckies can even fuse to become these undulating, multiheaded beasts spilling over land and sea. It's very extra, and not very energy efficient, but in Final Form, rubber duckies are unstoppable.

As terrifying as Final Form is, it's easily the rarest state for duckies. Usually, they're in their little, adorable rubbery shape -- the best one for conserving energy, both directly and in the "look at me I'm just a little guy and it's also my make day" sense. These birds are intelligent people, and some of them do have a sense of, if not strict humor, the absurdity of their existence. The playful ones can appreciate a little joke, a little play, and some limited-harm pranking. As long as they don't see you as a threat (or the buffet line), the worst you'll probably get from a rubber duckie doing a jumpscare into Hunt Form on ya.

It'd make a great haunted house setpiece.