Krafayis

The Krafayis are a sapient nomadic species of invisible creatures which travel from planet to planet and hunt in packs. Their world of origin is not known, and they are known to have a strictly hierarchical pack-based society, in which each group dominates a territory encompassing several star systems. They are also aggressive, non-compassionate creatures that will not hesitate to abandon a member of their own pack who isn't fit enough to assist in the hunts.

Biology
Invisible except under a few circumstances, Krafayis are fairly large quadrupeds with a beaked mouth, a crest of quills on the head and another set of long quills running from the end of their backs to the tip of their tails. Their limbs are armed with long claws and they engage in knuckle walking, similar to Earth's anteaters, presumably to minimize damage to their front claws. They seek planets with a nitrogen/oxygen based atmosphere, indicating an aerobic metabolism.

Their most notable feature is their invisibility, although they apparently become visible on at least some kinds of mirrors or other reflective surfaces. The only known individual who was capable of seeing Krafayis without a mirror was painter Vincent Van Gogh. It is implied that his unique view of the natural world may have been the cause of his ability to see what nobody else saw, indicating that the invisibility of the Krafayis might be based on psychological, rather than physical, principles.