Flounderling

On such an one I came All inadvertently, Splashing the shallows Where Atlantic ebb Was making bare The flats, As when one Wading Sets unguarded step Upon a lurking Flounderling.

At first flounderlings are thin-bodied but typically fishlike in other respects. Then suddenly striking anatomical changes take place, and the creature's behavior becomes fixed for an essentially two-dimensional way of life.