The Yithians, more formally known "The Great Race of Yith", are a species of body-jumping aliens from the Cthulhu Mythos, first introduced in the story The Shadow out of Time. The title Great refers to the fact they are the only species to have mastered a form of time travel.
They dwelt (or will dwell) on the planet Yith, which may be their homeworld.
The original bodies of the Yithians are never described, since they use their advanced technology to jump their consciousness through time and space into new bodies. The Cretaceous Yithians, who had escaped their own extinction by projecting themselves to Earth, used the body of a large invertebrate creature, who resembled a rugose cone topped by a pair of clawed appendages, a long trumpet (presumably a feeding organ) and a yellow orb-like sensory organ on a stalk, sliding around on a sluglike mucous foot.
The cone like creatures inhabited by the Yithians are asexual and grow children around the bottom of their bodies. Reproduction is rare as a Yithian typically lives for long periods of time. Young Yithians are brought up by there parents, though later familial relationships usually are not given importance. Yithians do however form bonds with the same generational groups.
The Yithians form a loose nation or league run by a small governing board. These governors are voted in by those who pass educational and psychological tests. The nation seems to be split into at least four distinct divisions each given its own board and control of its resources. These are a partly fascistic socialist system. Resources are rationally distributed. All industry and food production is done mechanically. Instead of meat the Yithians only eat vegetable and synthetic foods.
Because most labor is done by machines citizens have much leisure time. This mostly is used in scholarly and aesthetical pursuits. Technology progressed to a height due to the need of surviving the cataclysmic environs of the Yithian culture on Earth. Crime barely exists and legal procedures are carried out with complete respect to the criminal. Civil wars are common inside the Yithian nation, though it often fights the prehistoric civilizations around it, including the Old Ones. But more impressively a huge army is kept on hand with "camera like weapons which produced tremendous electrical effects." This army was to defend the race from the Flying Polyps that were trapped under the Yithian cities.
The topic of the Flying Polyps is the only taboo in the Yithian culture. The Polyps where originally masters over the conical race that the Yithians came to inhabit. When the Yithians projected their minds to that of the conical creatures the polyps were taken by surprise and the Yithians were able to trap them in their black ruins underground. The Yithians considered these polyps to be so evil that they keep an entire army ready to combat any escaping polyps and eventually flee the minds of the cone like creatures into the future.
The Yithians use their mind-projecting techniques to scout out the passage of time, allowing them to accurately predict their own extinction and avoid it. To do this, the mind of a specimen from a (usually sentient) species is swapped with that of a Yithian for a period of several years, so that the Yithian can record information from that time. The target specimen, if willing to cooperate, is allowed to freedom to roam libraries and communicate with other specimens until his time of return comes. The memories of living in the cone-body, however are suppressed before the specimen's mind can return, to secure the secrets.
When the alien Flying Polyps reemerged to doom the civilization of the Cretaceous Yithians, the race, who had long ago predicted this, mass-moved their species' consciousnesses into the "Coleopterous race", beetle-folk living in Earth's far future. It is beleived that due to this, the Yithians will exist until the very end of the universe.