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 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.
Information gathered by the Yithians is recorded in the great library at Pnakotus, now a ruin in the Great Sandy Desert of Australia, from which the Pnakotic Manuscripts are derived.
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.