Mysteries of Yith, Part 1

It's stated in The Shadow Out of Time that the cone-shaped bodies of the Yithians were not their original forms. They traveled by mind-transference to escape whatever was destroying their original homeworld, Yith.

It seems likely that Yithians were related to the beings of Yaddith (from Through the Gates of the Silver Key and the pre-Mi-Go inhabitants of Yuggoth (mentioned or implied in The Whisperer in Darkness and The Haunter of the Dark , because the three words sound like they could be different forms of the same root (maybe just meaning "planet" or "home world"); 'Yaddith', seemingly the oldest planet, since it seemed to have died before life on Earth amounted to much, perhaps being the original form; 'Yith' a worn-down elided form; and 'Yuggoth' a corrupted form. (We're never told about the form of the first Yuggoth species or the pre-terrestrial Yithians; they could well have been the Yaddith beings' 'queerly jointed' semi-insectoid semi-reptile bodies.)

We know (Through the Gates of the Silver Key) that Yaddith had space travel technology, and that they faced extinction by Dhole. Perhaps they left their planet near the end, scattering to many worlds. One group, guided by hints accidentally dropped by Randolph Carter/Zkauba and preserved in the race's records, headed toward Sol and established a base on Yuggoth, a planet more easily made suitable for their metabolisms than hot, high-gravity Earth.

Most of their colony worlds failed due to decline or being overtaken by greater powers. Yith, however, had developed the mind transfer technology before its final fall....

Another mystery is why the children of Yithians that had transferred their minds into cone bodies were born with Yithian minds and not whatever the original species had been. However, perhaps being raised in a Yithian culture was sufficient to make them functionally Yithians.