Gaki are invisible to the human eye, but some believe they can be discerned by humans in certain mental states. They are described as human-like, but with sunken, mummified skin, narrow limbs, enormously distended bellies and long, thin necks.
This appearance is a metaphor for their mental situation: they have enormous appetites, signified by their gigantic bellies, but a very limited ability to satisfy those appetites, symbolized by their slender necks.
The image was created from parts of the Scroll of the Hungry Ghosts (Gaki Zōshi 餓鬼草紙)
Late-Heian Period (Late-12th Century), National Treasure.