To change form - to shape one's matter as clay - this is no novice's trick, no flickering glowlight or rat-killing blue bolt. Only those with bowels soaked in oceans of magic arcana - and few are born with bowels so absorbent - can hope to master it.
There are exceptions, of course. Dragons, as is well known, possess such a talent innately. With no need for study they can, guided by some otherworldly intuition, change between humanoid and reptilian form. As for the other races, higher vampires (e.g.. bruxae) are known to flicker between states, yet given the understandable difficulties of conducting research in this area, we do not know if this transition constitutes an act of polymorphy.
The human race is, obviously, much more accessible to scholars of polymorphy. Their years of study have borne fruit in the form of a set of methods and guidelines for the identification, nurture and deployment of this talent. The most accomplished result of the careful application of these methods - Philippa Eilhart.