In Scottish folklore of this dangerous creature, it was said that the Boobrie is metamorphosed form of the Each uisge (which is a water horse-like creature related to the shape-shifting Kelpie) and the Tarbh-uisg (which is a species of water-bull also related to the Kelpie). This is both false and true; the Boobrie itself is a species of pure terror and other creatures know this. Shape-shifters such as the Kelpie, the Each uisge, and the Tarbh-uisg learn by copying the image of another apex predator at an early age for protection or to strike fear into an intruder. Kelpies are known not to copy a Boobrie's image but, young Each uisge and Tarbh-uisg do for these reasons. Its a survival strategy used by many monsters and everyday animals called camouflage.
The illustration above is what a Boobrie looks like appearence. Terror Bird size and appearence, Stork's structure, and the Loon's markings.