We have existed since the dawn of time. We watched as the great pyramids were built. We watched as the Romans crucified the Christian Messiah. We watched as Caesar slaughtered the Gaelic. We watched as this world grew from a screaming infant into a raging psychotic. We have watched all this and more while you have lived and died around us, for we are the immortals. The long lived, the undying. And you, my precious, are our prey. For, above all, we are Predators...-Ishtar, Queen of the World
and Goddess of Love and War
The simplest term to bestow upon them would be 'vampire' for they are drinkers of blood, but the Predators are not like any other vampire you've imagined. Unlike the vampires of literature, myth, and legend, the Predators are living, breathing creatures. Their bite is not virulent and they can not 'embrace' a human being into their fold. They reproduce the same way as you or I, be it with a mere human or another Predator, and the sun is not always something to be avoided on penalty of an agonizing death.
They do not know how or why they are the way they are, but current cerebration in this age of science leads some to believe that they are the by-product of some sort of venerable virus that plagues their body's systems. It is this that gives them their awesome super-human abilities, but this empowering virus (if that is indeed what it is) has it's own demands. Blood. How and why they require blood is also up for debate. Believers in the virus theory think it is what the virus itself needs to flourish, as a virus can not reproduce itself without a host cell to first invade. But this is, of course, all speculative.
Not to surprisingly, these virus theorizers haven't a single clue as to where this proposed virus of their's came from. Some believe it to be merely an evolutionary mutation, while others think it might be of an unearthly origin.