Metamorphs

Metamorphs, or shape-changers, are a familiar sf concept. Poul Anderson, as always, does not merely accept the idea but reasons about it logically. In his The War Of Two Worlds (New York, 1959), no such beings have evolved naturally but one Sirian race, having mastered biology, uses manipulation of individual genes and forced growth to develop artificial mutations as metamorphic spies and infiltrators with:

pigmentation cells;
flexible tissue;
a calcium system able to grow bones and teeth in seconds probably around a cartilaginous base;
a nervous system able to control these many intricate details;
an equilibrium that can be upset only be electric shocks because nerve currents are electrical.

Thus, Anderson not only rationalizes protean powers but also extrapolates a logical weakness for such organisms. And he addresses not only their biology but also their sociology. Having helped their creators to conquer the Sirian system, they are feared, ostracized and forbidden to breed so they steal a spaceship and set out to colonize the Solar System, replacing high officials of Earth and Mars and their families, then engineering a war that weakens both planets.

As with Frankenstein, we are shown a monster, then made to feel sorry for it.