They are organisms which could evolve from the groups being in reality extinct, with no actual descendans. Like for example therocephalians, non- avian dinosaurs, pterosaurs, amonites and so on.
Oh really, so I must add it not only this - also creatures from parallel universes ( not like simply alien which meant to live somewhere in our real this here universe) where evolution took different directions. And one more thing - organisms which could exist in the past but we wil never know them because their fossils didn't made it to present.