Part of the fantasy world I am developing: a vampire's enhanced "beast form".
Among the other shapeshifting capabilities of vampires, the ability to shift into a bestial form is the most drastic. The extra mass gained (approximately 2-3 times their original mass) is not actual mass, but rather magical energy supplementing their cells. In addition to increased speed, strength, and resistance to damage, a vampire's beast form shuts down most of their higher brain function, including their ability to recognize faces and tell friend from foe. Transformation is incredibly painful and staying in the form for too long may result in them getting "stuck" and going feral. Because of these reason, this capability usually reserved for dire circumstances.

A vampire starved of blood will eventually go into a "blood rage", involuntarily transforming and slowly losing higher cognitive functions. If they fail to feed sufficiently, they will go feral, losing their intelligence and staying in this form for the rest of their lives. Ferals become savage beasts, acting on animalistic instinct alone.