The most iconic of all the debuffs, the CC to rule them all and sometimes the funniest thing to happen all battleground is a mage's polymorph spell, or "sheeping."
History is riddled with tales of humans being turned into animals for the caster's convenience. The most famous of them all is a Brother's Grimm tale: "The Frog Prince." There are several replications of the tale, but the general idea remains the same. You never know if that's just another sheep, or a prince inconveniently polymorphed into one.
A more literal interpretation of real life polymorphing and other video game shifting could be extracted from our views on evolution. Nothing quite this time efficient happens visibly, but on the nano level it's easy to see rapid morphing and mutating.
Viruses are constantly evolving to survive, and the majority of changes occur to their appearance. The more a virus differs visually from it's predecessors the more likely it is to go unnoticed by pathogens and thus rapid evolution, or morphing.