Krafayis

The Krafayis were a species of space-faring carnivorous predators.

Biology
The Krafayis are a race of bird-like reptilian creatures with an unknown origin. Their life is similar to one of a birds or vultures. They are raised and then voyage forever attacking and defending its fellow race, as well as the local races on the planets they land on. It is invisible to the naked eye but could be seen by Vincent van Gogh for unknown reasons, though it is suggested it is linked to his depression. The Doctor used a device that can identify species when it is seen in a mirror, but whether the species is only visible through reflections or the device is actually designed to see these kinds of aliens is unknown.(DW: Vincent and the Doctor)

Life Cycle
During their early life they are nurtured in a similar way as a bird would be. After several hundred years, they embark on a never ending voyage with up to three thousand other Krafayis journeying across the universe and devouring creatures they find. They are ruthless, and will not return for those Krafayis left behind on a planet they have hunted on, leaving them to fend for themselves. It is hinted that they leave the weak behind, such as the Krayfayis left on Earth due to its blindness. (DW: Vincent and the Doctor)

History
The Krafayis have travelled the universe since the late Dark Times and have a unique defence system where they cannot be seen by the naked eye. They have voyaged across the universe in packs and devouring many life forms. One was left behind on Earth and was scavanging on humans. (DW: Vincent and the Doctor)