Polymorph is a slang term used by Magic R&D to describe the following effect: "Exile target creature. Its controller reveals cards from the top of his or her library until he or she reveals a creature card. That player puts that card onto the battlefield." This effect is named after the card Polymorph from Mirage.
The effect is currently primary in red and secondary in blue. For a long time, this ability was a blue thing. When R&D was looking for more places to expand red, they decided to slice up the polymorph pie. Blue gets transformations where the outcome is known and red gets them where it is not. The idea being that blue only wants to transform something when they know what they're going to get, whereas red is willing to take a risk. Red will also do large chaotic effects where many things change all at once.